A hair is a short tag of line with a stop or a band, so the bait sits off the hook. When a fish sucks the bait, the hook follows. It is standard on method, bomb and boilie fishing.
You don't need a hair for a single maggot on a canal. You do need one for a wafer in a method mix.
Components
Hook with an eyed or spade end, a hair of the hooklength material or braid, a bait stop or band, a needle.
How to tie it
A knotless knot is the usual: hair the length of the bait plus a bit, stop on, bait on, whip the knotless knot, tag away from the point.
Common mistakes
A hair so long the bait waves. A hair so short the bait sits on the gape. Nailing the bait on the shank and calling it a hair.