Definition of softplaying
Softplaying is the act of deliberately playing passively against a specific opponent. Most of the time it is collusion, meaning that the two players who are softplaying know each other and do not want to take money from each other. If you are caught softplaying in a casino, you could be warned and probably kicked out of the poker room if it happens again.
Softplaying, on the edge of legality
Usually, you should play with the intention of winning as much as possible. When you softplay a player because they are your friend, it is a type of collusion that will put other players at a disadvantage. For example, if you are in a tournament and would normally steal the blind from the player in the big blind and you don't because your friend is sitting in that seat, it can frustrate many players. It's not always easy to prove that a player has softplayed one, but be aware that if you do, you could be negatively affected.