Pairing is when two programmers work together on a problem. We do it a lot at DBC.
Since we'll be pairing with you, if you don't know something, like some fact or detail, that's fine. You should really focus on how to think and talk about the challenge (the technical problem), ie. how to explain what you're thinking, follow what we do together and talk about it, and be able to recognize when we've succeeded!
Most of all just try to relax and enjoy working with us through this process.