In BDSM 'limits' are activities that people feel strongly about. Often strongly enough that they are prohibited. BDSM participants typically discuss and negotiate their limits with partners before any activity takes place. It’s essentially a more comprehensive consent conversation. There is a distinction made between hard and soft limits:
A hard limit is something that must not be done.
Eg. "Poop is my hard limit"
A soft limit is a little more difficult to define as it can have a number of different meanings even for one individual.
Soft limits are activities that aren’t strictly prohibited but may require specific or strict conditions, or an especially cautious approach. Or it could be something that a person is hesitant about but not opposed to, so it may still happen should they decide to give their informed consent.
Eg. "Poop is a soft limit for me – I only will consent to being pooped on; on my stomach, after sunset, and after I have had an orgasm"
Requirement Limits are sort of the opposite of soft and hard limits. A requirement limit is something that without which the person will not participate in any activity.
Eg. "Poop is a requirement limit for me – I will not play with you if poop is not involved"