A river is a natural watercourse, flowing toward or from another body of water, such as an ocean, lake, or another river. They often meander, due to erosion factors.
Rivers appear in many video games, most often as landscape features. They don't have to be just a part of the scenery; they can also serve as a game mechanic. For example, in the Mario Party mini-game "Paddle Battle." Furthermore, rivers can serve as boundaries between different zones in a play area.
In some MOBAs, notably Dota 2 and League of Legends, a prominent location on the map is colloquially known as "the river." This is because it is a river.
Many references are made in these games to the river. For example, if one player doesn't want their teammates to go into it, they might say "don't go into the river, Ursa and Lina are there and will gank you."