The main level League of Legends is played on is called "Summoner's Rift" and it's based on the original WarCraft 3 map, Defense of the Ancients: All Stars, and StarCraft map, Age of Strife. The map is divided into three main paths, called "lanes," where troops ("minions") automatically spawn and travel down.

Each lane is guarded by a series of powerful towers that shoot magic missiles at enemies ("turrets"), leading back to each teams' base where troops are made. At the very back is a Nexus, guarded by even more towers. The goal of the game is to destroy the other team's Nexus. Players each control a champion, a unit that acts independently of the automatic minions. Champions are much stronger than minions, but start the game much weaker than turrets. As champions defeat other champions and kill minions, they level up and get gold to buy magic items with. Eventually, champions become strong enough they can overpower turrets and push into the enemy base to attack the Nexus. It's important to have champions from your team to guard each lane, otherwise enemy champions will overwhelm your team's turrets.

Between each lane is a large wooded area, referred to as "the jungle." On the WarCraft 3 map, it was a literal jungle, so the term stuck around. This wooded area is filled with neutral monsters players can slay for gold and experience. Champions moving between lanes to defend often have to go through these areas, leaving them open to getting ambushed by a player whose job it is to patrol and explore the jungle, called a "jungler." Dividing the two halves of the map is a river with large monsters worth lots of gold which junglers often fight over.

Back to "top, mid, bot." This refers to the three paths minions travel. One player guards the middle lane and has to play a champion capable of protecting themselves while able to travel between lanes or hop into the jungle. This is generally a wizard with large area of effect attacks that can wipe out minions waves easily.

In the bottom lane are two champions: an "attack damage carry" and a support. ADCs are ranged champions that deal damage through automatic attacks and don't need any resources to do so. They are vital to making the final push on the Nexus late-game because often a team will be weak after wiping out the other, so ADCs help bring down turrets through auto-attacks. To stop the ADC from being the dumbfuck "I'm the main character" people that they are is a support champion. These are usually healers or "tanks" that act as bodyguards for the ADC. ADCs are especially prone to getting killed by junglers and mid laners who come to the bottom lane for ambushes.

The reason mid and bottom are always set up like this is because a neutral dragon spawns in the river early in the game junglers will fight over. Both lanes often assist their jungler because the other team is doing the same thing. Slaying the dragon early can provide enough gold to snowball the rest of the game. So in addition to protecting their turrets, they're also protecting that section of the river.

Lastly, this leaves top lane. Top lane is the loneliest lane because most of the action happens at the bottom. Top laners spend most of the game on their own, so they need to be able to win duels. This also means they're easily attacked by junglers, since their isolation can make for easy kills. They have their own jungler and mid laner to help, but top laners need to be able to escape easily or fend off multiple attackers until help arrives. Thus, top is usually filled by strong fighter/brawler champions or designated duelists.

Of course, all of this goes out the window on the Howling Abyss, a level with only one lane and no jungle. Just ten peopled crammed into a tiny ass space throwing snowballs at each other.