The hairy, wire-like thread that is found at the base of a mussel shell, and attaches the shell to rocks, is known as a beard. Remove the beard before cooking using sharp kitchen scissors.
Some shellfish, including mussels and oysters have hair-like strands attached to their closed shells. These strands are known as beards, and the shellfish are sometimes referred to as 'bearded'. The beards should be removed with a sharp knife or kitchen scissors before cooking.