A fid is sort of like a fat knitting needle that is usually made out of wood and tapers down to a sharp point. It is used to spread apart strands of line for splicing or to open stubborn knots.

If you have a stainless rigging knife that has a knife on one side and a long pointed thing on the other side, that pointy thing can be used as either an awl or a fid.

(When dealing with rope, a fid is also a unit of length defined as 21 times the diameter of the rope that is being worked with.)