His nobs is 1 point for holding the jack of the same suit as the starter card. It is scored when hands are counted, and it counts in the crib as well.
After the discards, a starter card is cut and turned face up. If your hand holds the jack of that same suit, you score 1 point for “his nobs” (also written “one for his nob” or “one for the right jack”) when you count your hand.
Example: the starter is the 7♥ and your hand contains the J♥. That jack is worth 1 point on top of whatever fifteens, pairs and runs the hand makes. A J♠ in the same hand would score nothing extra.
The dealer’s crib is checked for nobs too, so a jack thrown into the crib can pay off for the dealer.
Don’t confuse nobs with his heels, which is the dealer’s 2-point bonus when the starter itself is a jack.