Here's an excerpt from my 'rags to riches' guide on the basics of league.
The Meta:
The meta is the basic framework in which pro players chose the very best way to play League of legends.
The meta shifts because as new updates settle in, what we knew before as the meta now is outdated. Nothing stays the same.
There are 5 roles in league of legends.
=Adc = Top = Mid = Jungle = Support =
There are 4 lanes (Technically speaking).
Top, Mid, Jungle, Bottom.
There are 5 players.
This means to maximize gold efficiency, somebody needs to give up their bank account.
If there were 5 people who only had 4 cars, you don't split one in half and share. You take a ride in one of the cars. That's what the duo bot is, a way to mitigate gold loss while still having a viable 5th teammate.
The reason WHY they are bottom though is the real question, why not mid or top?
Simple, the bot lane mid would be responsible for roaming, not like the assassin mid is now . The primary reason why they are bot is because they are closer to dragon and have a longer lane to fight in, versus mid lane who usually has gap closers. They are relatively the same distance from dragon, and it wouldn't hurt to swap laners. HOWEVER, a smaller lane for an adc reduces all in potential because of the safety from tower, and a mid laner bot will stop all possible roam routes top.
Why are mid laners mid?
Typically speaking, mid laners are usually assassins. They pop in, and they pop out. Kha zix , Katarina, Fizz, Jarvan, Diana, Lee Sin are all potential assassins that can really 1 combo a target. Because they have a gap closer and fast burst, it's only right to put them mid, where tower will do massive damage early if you're mot careful and its easier to run back to.
Why are top laners top?
Usually, top laners are champions that do well later, not so well now.
Exceptions to that would be bullies like Riven and Jayce who focus on knocking their laner out like Nasus or Ryze. They are here because they need a long lane to farm and fight in for the majority of the game. Before their powerspikes, they will probably afk farm top unless they have tp.
Why are junglers jungle? Why not 2 or 3?
Junglers are there to carry their team through the early game, they are expected to balance out farming the jungle and ganking a lane. Having 2 junglers certainly does alleviate the pressure on having to gank 3 lanes, but you will have to gank a certain lane twice as much. Yes, bot lane. The 2v1 lane that is stuck without a support because there are two junglers, you will have to 'gank' him so much that you tell yourself 'might as bee a suport' because if you don't and you hit tab. You will see 1 dorans blade at 20 minutes because he was stuck at tower all game.
END.