There are often two types of limits defined, hard and soft. My own personal interpretation of those is:

hard: don't go there, ever, don't think about it; complete nono (but I totally understand what annie is saying about the dom needing to find out the boundary of some of those limits)

soft: scared, don't think I'd like it, makes me go eugh, don't want to do it - but accept exploration and pushing of them (gradually) as part of the learning process. Some may become pleasures and actively desired; some may become things that I will do to please, but without personal pleasure; some may become hard limits.