Nib is an expression used by woodworkers to describe the rough texture on the surface of wood that has received its first or 'sealer' coat of finishing lacquer, paint, varnish, etc. As it dries, the liquid finish tends to lift the minute cut ends of the wood fibres. Once the sealer coat is dry these fibres become crisp, giving the wood's surface a feeling rather like the face of a man who didn't shave this morning. This is nib.
Dust that wasn't properly cleaned off the surface of the wood before the application of the finish can also contribute to nib.
Once the sealer coat of finish is thoroughly dry, and before the next coat is applied, the nib should be removed. This process is called de-nibbing. If de-nibbing is neglected a high quality finish will not be achieved.
The best abrasive for de-nibbing is a silicone carbide (self-lubricating) abrasive paper. A very fine grade should be used - typically P220. Gentle use of the abrasive is advisable, so that the nib is removed without excessive abrasion or removal of the sealer coat.
Before application of the next coat of liquid finish, ensure that any de-nibbing dust is removed from the surface. If, after the second coat, further nib can be felt, then additional de-nibbing and a third finishing coat will be required. Note that nib is much easier to feel than to see, so the fingertips are your best guide in making the decision to carry out further de-nibbing.