honest nooblike question

It doesn't.

mmr is a statistic that defines your player level, more so than elo or rank but these are all related. Generally it doesn't impact gameplay and isn't something you need to take into account when getting into a game.

Depends on what you want to know. MMR in its simplest form that determines who you play against.

By winning games you gain MMR, by losing games you lose MMR.

By losing games against lower ranked people you lose more MMR, by losing against higher ranked people you lose less MMR. Vice versa for winning.

Normals and Ranked have 2 separate sets of MMR.

Matchmaking is based on a teams total MMR. This means that as long as the number evens out a team could theoretically have 1 diamond, player 1 low plat player, and 3 high gold players, whilst the enemy team could have all mid plat players.

MMR affects matchmaking and also determines how much LP you gain and lose upon winning or losing ranked matches.

Having a higher MMR than the average of your current rank matches you to play against higher ranked players which nets you more LP for a win, having a lower MMR than the average of your current rank matches you to play against lower ranking players which nets you less LP for a win. Vice versa for losing.

Since everyone has given the simple "how it applies to League" answer, I guess I can give you a link to something more in-depth.

MMR is essentially an adaptation of the Elo Rating System from chess. This is why most people call rank "Elo". It does 2 things: Helps determine the average skill level of a player after so many matches. Determined how much MMR you gain or lose from wins and losses.

Losing to less skilled players means a greater MMR loss and vice versa.

If you want the maths for it, you can always check out the wikipedia on it. https://en.wikipedia.org/wiki/Elo_rating_system

The League System and how Riot calculates MMR uses slightly different variables, but overall the same equations.

In addition to what others have said, MMR has two special interactions with the Ranked system.

First, if your MMR is very high for your rank (like you consistently win matches against higher MMR teams), it is possible for you to skip divisions (i.e. win your Silver4 promos and go straight to Silver1).

Second, it's not normally possible to demote tiers below Master (i.e. Silver demoting to Bronze). In order to demote, your MMR has to be at the expected MMR level of someone an entire tier below you. So to drop from Silver5 to Bronze, your MMR needs to be Bronze5 level. Because of this, tier dropping is fairly uncommon, and is usually a sign of something like not playing for a long time or getting boosted.