Some great answers, but none of them ELI5, so I'll have a go.

When electricity passes through a non-metal, it's like trying to push a person through a wall, the person just bounces off. When electricity passes through a metal, it's like you put lots and lots of doors in the wall, so the people (electricity) can pass through it easily. Different metals have different conductivity (more or less doors).

Superconductivity is like taking the wall away completely, 100% of the people can freely pass the threshold. But, so far, we've only been able to make superconductors that work at very, very, very low temperatures; or very, very, very high pressures. Of course, it's not viable for our computer or electric cables to be cooled that much, or pressured that much.

In our modern world, with so many devices running on electricity, we lose lots and lots of energy & money to resistance (those pesky walls with not enough doors). If we had a superconductive material that works at room temperature, and normal pressure, it would mean we can send electricity around the world with very small amounts lost to resistance; it would mean our devices would become incredibly efficient; and likely lead to the development of incredible new technology.

A working room-temperature superconductor would have many, many applications.

Just for example:

Superconductors are currently used in MRI machines for medical imaging. They require cooling to extremely low temperatures. Today's MRI machines consume liquid helium to keep the superconducting components cold enough to function. A room-temperature superconductor would make MRI imaging vastly cheaper and more sustainable. (Helium is a limited resource; the planet ain't making any more of it.) It could even be cheap enough to replace medical X-rays.

Superconducting coils could be used in power systems, including generators and power storage systems, to increase their efficiency well above what can be done with today's copper coils.

It's less likely, but superconducting wires could be used in electronics of all sorts to reduce energy consumption, improve features like wireless charging, etc.

I think it's also very interesting from a 'pure science' background. Superconductivity isn't 'perfect conductivity' - it isn't that we've found ways of making normal resistance less and less until eventually we made it to zero. Instead, there's certain materials that, as the temperature falls / pressure rises, that all of a sudden the resistance just disappears completely. The electrons pair up in a different way from usual, and we see different properties, like completely ejecting all magnetic fields.

They tend to have a 'breakdown current' above which they stop superconducting, so it's not like they're the instant answer to all of our power distribution woes. They could help in places, but they also tend to be amazingly expensive to manufacture, so they're no magic bullet. Their magnetic properties might enable us to make things like rotating hard disks with truly absurd storage capacities - you wouldn't need much superconducting material for each one.

We've had superconductors that operate at liquid-nitrogen temperatures for quite a while, which makes them easy enough to study in a laboratory. Having room-temperature ones makes it even easier, and might let us understand them even better. This one sounds a bit impractical for wide-scale use (it's a powder) but might let us develop more useful ones.

My attempt: ambiant temperature superconductor would be like when we went from lamps to semi-conductor. We were doing many things already with lamps, but semi-conductor allowed for incredible miniaturisation and efficiency. They allowed the invention of computers, Internet and smartphones.

We already do many things with magnetic fields, but ambiant temperature superconductor would be a big technologic jump.

Many things would rapidly become much more efficient, but we have no idea yet of the full potential of this technology. It could be helpful for quantum technologies and fusion reactors for example.

It"s a big deal because hoverboards and hovering cars like in the movies

Superconductors can transmit energy with no loss. However current superconductors require very low temps.

Room temp superconductors would open the door to much more efficient transmission of electricity in everything from the electric grid to your home computer.

Now whether the LK-99 can be replicated reliably, verified to be superconducting and what the properties of LK-99 would be at scale would dictate how it could be used (is it malleable? does it wear? are there things that cause it to lose its superconductive properties?).

Bear in mind the paper(s) has not been peer reviewed, and nobody has replicated the material, and the video they released is not very convincing. A room temperature superconductor would be huge news, if they actually created one.

Biggest one's I've seen pointed out are how it can affect quantum computing technology and how it might help us make nuclear fusion more attainable.

Quantum computing can be broken down by it's words. Quantum being a state that fluctuates, meaning it's not always one thing or another. Computing, well it's computing. Quantum computers then would be computers utilizing quantum mechanics in a way that I understand it as, if things are currently ran where a number can be one or zero, it could now be one AND zero at the same time. Currently this takes massive amounts of cooling as a computer chip trying to skirt the laws of physics probably runs a little extra hot. This tech could allow for massively reduced heat production once someone figures out how to factor it into the design of the parts that normally get hot. You gotta be a smart 5 year old to get it but that's my best attempt.

For nuclear fusion, the chemical reaction that occurs generates so much heat that, to contain the reaction to a size we can manage, we use more energy keeping it cooled and contained and going than we gain from harnessing it. If we can reduce the energy needed to keep it going, we could potentially have a less volatile and more sustainable source of energy that we've dreamed about as a species since we figured out what the stars really were.

For example, you can resolve a lot of blockers of scaling quantum computers based on superconductive qubits.