MMR is a hidden attribute that we only use for matchmaking. If you win a 1v1 match, you get MMR points. If you lose a 1v1 match, you lose MMR point streaks. And if the match is a draw, both players gain a small number of MMR points. If you are on a winning streak and win multiple matches in a row, your MMR gain is boosted so you can quickly get to a more suitable level. And the opposite is true; if you lose several matches in a row, you will descend more quickly to a level where you have a better chance of winning.
You continue to collect MMR points throughout the season. You will only be matched with opponents within a certain range of your MMR level. However, if the opponent has a slightly more MMR points than you and you win the match, you will get a small MMR bonus for beating a stronger opponent. Similarly, if you lose to a player with fewer MMR points than you, you will get an MMR deduction so that you will be matched against an easier opponent in the next match.