BBC actually has a decent explainer.

If Putin's description is correct, the missile is at the upper edge of the definition of hypersonic, and few things can achieve this.

Speed is important because the faster a missile travels, the quicker it gets to target. The quicker it gets to target, the less time a defending military has to react.

A ballistic missile generally gets to target by following an arcing path up into the atmosphere and a similar one down towards its destination.

But as it descends, it picks up speed and gains kinetic energy, and more kinetic energy gives it more options. This allows it to manoeuvre down towards the target - by performing some kind of defending wriggle - that makes interception by surface-to-air missile systems (such as Ukraine's US-built Patriot defence missile system) particularly difficult.

https://www.bbc.com/news/articles/cvg07zw9vj1o