A caveat is a warning. When someone adds a caveat to something they’re telling you to beware — maybe what they’re telling you comes with certain conditions or maybe there’s something dangerous lurking.
When your new friend gives you directions to her house, and then says, “The caveat is that when it snows the driveway turns into an ice rink,” she's warning you that your travels could be dangerous. Caveat is also a legal term for when a lawyer asks for a break in proceedings. If a lawyer issues a caveat, she's filing a formal notice to suspend a trial until her client gets a hearing.