Limits in BDSM refer to the activities either a sub or dom is unwilling to partake in for a variety of reasons. Discussing activities you would not do is as important as discussing all the fun and kinky stuff you would do with your partner. This is usually divided into two; activities a person will never be willing to partake in, and activities a person might consider upon discovering more about themselves but are not willing to try at the moment. Wikipedia. 

Take a moment to check some examples of some people's soft limits and hard limits list on quora.

There are also ‘requirement limits’. Although important, they are not talked about often. They are activities each party requires of the other before, during and after play. For example, the sub requesting her hair to be pulled, asking for aftercare, e.t.c.

There is also another type of BDSM limit known as ‘temporary limits’. They are set when you go into a play space that is not yours or you are not familiar with. These limits are valid for only the time you play in the temporary play space. They could be as simple as no blindfolds in the play space, or no penetration while in the temporary play space. 

Especially for beginners who are new to kink activities, figuring out your BDSM limits might not be as easy. You still have to set them anyway. Check out a few tips that can help you discover your BDSM limits:

• Think of things you know you would never do. These are activities that have an adverse effect on you emotionally, psychologically, and physically. For example, activities that disgust you, stretch your physical capabilities or trigger a mental breakdown. 
• If it is a casual play partner, consider if there are limitations to the activities you want to engage in with them and if you would like to reserve some activities for your long-term romantic partner. 
• Do your research. Check out the different kink activities people engage in.
• Attend dungeon parties so you can learn how other BDSM partners act in their play space.
• Speak to a professional in the kinky lifestyle that you can trust.
• Fantasize about engaging in the different kink activities you have discovered.
• You may also try some of these activities in your solo play to see how they feel. For example; self-bondage. 
• Using a checklist also helps you to discover, properly articulate your limits and communicate them with your partner. 

N.B. You need to be wary of people who try to help you ‘explore’ or ‘learn’ your limits without taking enough time to know you. That can be a red flag. Well-meaning and experienced kinky individuals take their time to know a newbie and can now help you explore after plenty of communication. 

You can create your BDSM limits here. 

In situations where limits are crossed unintentionally or the limit that is crossed is minor, the erring party could be issued a serious warning not to do it again. 

However, if such a situation occurs again or major breaches of boundaries occur, you should terminate the relationship.

Also, ensure that you discuss the consequences of breaching boundaries before you get involved.

Having learnt everything about limits, sometimes, one may cross a person’s limits unintentionally. This is why it is important to discuss limits before engaging in any activity so you know the activities you should not pursue during play. Limits are for the benefit of everyone involved. Do not forget to stay safe and always remember that BDSM activities must be safe, sane and consensual. Have fun!