Definition of softplaying
The softplaying (which could be translated as "play softly") is the fact of voluntarily playing passively against a specific opponent. Most of the time it is collusion, i.e. the two players who know each other and do not want to take money from each other. If you are caught doing softplaying in a casino, you could receive a warning and possibly be kicked out of the poker room if it happens again.
Softplaying, the legal limit
Usually, you should play with the intention of winning as much as possible. When you play softplaying against a player because he or she is your friend, it is a type of collusion that will put other players at a disadvantage. For example, if you're in a tournament and normally you would steal the player's blind on the big blind and you don't do it because your friend is sitting in that seat, it can frustrate many players. It is not always easy to prove that a player has soft-played one, but be aware that if you do, you could have negative repercussions.