BDSM is an initialism for bondage/discipline ("BD"), dominance/submission ("DS"), and sadism/masochism ("SM"). It means some kinds of sex play, sometimes these are called kink or fetish. They all have to do with trying to get sexual pleasure out of things that are often painful or upsetting. People try to do this in a safe way by agreeing on a "safe word". If someone says this word the whole play stops. This is to stop the play from going too far and causing real physical or emotional hurt.