Fixing a worn spot in fabric by replacing the threads. General sewing term.

the act of mending a hole in a garment with crossing threads

Darning is a sewing technique for repairing holes or worn areas in fabric or knitting with needle and thread alone. It is often done by hand, but it is also possible to darn with a sewing machine. Hand darning employs the darning stitch, a simple running stitch in which the thread is "woven" in rows along the grain of the fabric, with the stitcher reversing direction at the end of each row.