Legends of Runeterra is the new free-to-play card game from Riot Games that launched an open beta in early 2020. There are several modes within the game, but most people will be playing ranked mode. Players can move up the ranks by earning League Points for each ranked win.
Winning ranked matches grants league points, and losing ranked matches takes league points away. After you reach a certain tier, you cannot lose the tier no matter how many games you lose. The only way you can lose rank is at the start of a new season.
Every player starts each new season fresh, though there is likely going to be some sort of hidden MMR so that Master ranked players won’t be stomping Iron ranked players in the beginning of every new season.