Basically just a list of topics the DM and the players might be uncomfortable with or just not tolerate.

A lot of people use 'lines and veils' where a line is a hard no and a veil is fine as long as it's happening offscreen.

As an example, as the DM I would have sexual assault down as a line. I just don't want it in my games. A veil would be something where it can happen and be referenced but not occur in front of us or be overly dwelled on. I'd have sex in general as a veil as well as things like torture. I'm just not comfortable enough with those topics to roleplay them actively.