Limits are restrictions to what forms of play are accepable in a D/s relationship.
Often limits are divided into two groups, hard and soft limits. Hard limits are things one of the partners is absolutely not willing to do. Soft limits are limits that can be pushed. Usually that happens only after previous agreement.
Limits of a submissive or slave are often viewed as a bad thing. A slave is view as ‘better’ when he has fewer limits. This is the main reason for a submissive to engage into pushing limits.