I'll fill you with my challenger-level experience, minion.
There is +120 champion in the game. Each champion have 4 abilities, 3 normal (with cooldowns) and one ultimate (with +1 minute cooldown, some has 3 minutes if they're overpowered). Each team has 5 players, for 5 different roles: Tank, Jungler, Mid-laner, ADC, and Support.
Tank is that fella on your team that can take a lot of damage, he's the one who's always on front blocking damage for other squishy team members such as your ADC and Mid-laner. Jungler is that fella that helps other lanes to win their lane by "ganking" their lane. Mid-laner is an Assassin or Mage role, rarely Fighter, but still counts. Mid-laner job is to sit in the middle of the fight, not in front nor in the back, and assassinate their ADC or any squishy other target he can reach if the ADC is positioned far. ADC is a short for Attack Damage Carry, It's the most important role on your team, basically, in the first 20 to 25 minute of the game you farm minions to get enough gold once you get enough Items you can then start doing well in team fights, because your auto-attack can deal tons of damage, but the ADC champs are rather squishy so they're protected by other team members. Support is that dude that make plays mid-game where they land CC (Crowd Control) and the team follow up on them.
There is tons of Items you can buy in-game, It take literally few games to start learning what Items to buy. There is tons of items, yes, but more than 70% of them are crap, everyone uses the other 30% left in the game which make it easier to learn how to Itemize your build.
That's the short version, if you need to learn more start playing the game yourself or watch live stream on Twitch.
