A safe word is exactly what it says, it 's a safety net. A safe word is something a sub can say at any point in a session which essentially means "stop everything right now this is too much for me". It should be a word that isn't used in normal conversation, i.e. 'stop' is not a good choice of safeword. So if a sub is in the middle of a scene and decides something hurts too much or they're too scared to continue they would say their safeword. Any Dom worth their salt will then stop what they're doing and spend as long as it takes talking to the sub until they find out what the problem was and what the subs feelings on the situation are and reassuring them that they are safe.

The idea is discussed at some length here http://www.bdsmlibrary.com/forums/sh...light=safeword

Hope that helps