What it is

An eligible receiver is a player who can legally catch a forward pass. Running backs, wide receivers and tight ends are eligible; most linemen are not.

In a game

The offense must keep seven players on the line of scrimmage, and the two on the ends are eligible. Players in the backfield are also eligible.

More detail

A lineman can become eligible by reporting to the referee, and wearing an eligible number. This gives the offense a trick, for example with a tackle-eligible play. An ineligible receiver downfield is a penalty.