In cribbage, nobs is the name given to the Jack of the turn-up suit. That is, if the turn-up card is a four of Diamonds, then the player holding the Jack of Diamonds scores an extra point in her hand, known as "one for nobs" (sometimes "knobs") or "one for his nob" (or sometimes "his nibs"). This is a very old term, which probably dates back to the origins of cribbage. What is a nob? The word is British English slang for an important person, so perhaps this is why.
The nob Jack (or "right Jack" as it's also known, is essential for scoring a perfect 29 hand, and also affects discard strategy - you should try to avoid throwing a Jack to your opponent's crib if possible.