Hard limits are something that must not be done under any circumstances. It’s a line that cannot be crossed. Hard limits tend to be activities considered more extreme (for example, activities involving bodily fluids), but can be anything you really, really don’t want to do. These are completely individual and should be thoroughly discussed before any BDSM activity.
There are other types of limits as well. A soft limit is something that a person might consent to, but only under certain conditions. Soft limits might be activities people find appealing but are nervous about, or know they may find uncomfortable but are willing to try if the right approach is taken.
Of course, you may not know what your limits are. It can be super intoxicating to try new things, but it's difficult to create limits and boundaries if you don't know all of the activities and terminology related to BDSM. Some tools that can help are BDSM checklists or BDSM contracts. You can even find sexual interest worksheets and lists of BDSM activities online, and these can form the basis of exploring what you might be interested in as it relates to BDSM. I've linked some of these here for you!