First of all, we need to know that League of legends is a 5 vs 5 game.
There are 5 possible lanes that you can choose.
Now, I'll talk about the Mid lane.
The Mid lane is the middle lane, as it says in it's name. In each lane, we have especific champions, but you can use the champion that you want.
A Mid laner it's an important piece in the game, he/she has to farm, kill and sometimes carry. They have the most part of the damage too (it can be AP or AD but it's recomended to have an AP mid laner if there is no AP in the team.), so if they die in the team fights or just go bad in their lane maybe the game it's lost.
I'll talk later about the champions and their principal lane.