A sock is a garment of soft material covering the foot and part of the leg, generally worn under boots or shoes. It may be any length from ankle to above the knee; there is no clear dividing line between very long socks and stockings.
A sock is a convenient item to use for a stuff gag.
School uniform fetish
Socks are used in school uniform fetish like schoolgirls. An authentic uniform should include socks; for girls' uniforms these are often white, for boys' these tend to be grey. In either case, they should be long enough to reach the knee, possibly with enough to turn over at the top.