Drains are an entity, ostensibly for water, which raises the player's gravity. They are mostly used to make certain areas harder to swim through, as if the player is getting sucked down. Drains can be used on land as well, and will still raise the player's gravity; making it harder to jump. It is considered bad form to overuse them, as it makes the player spam the jump key while trying to swim.
In previous versions, it let the player jump even after falling, but not after jumping; however, this is no longer true in SE.