The term "ineligible receiver" refers to any of the five offensive linemen. These players are not allowed to catch or even touch a forward pass thrown by an offensive player.
Offensive linemen are not allowed to receive the ball on forward passes. The numbers on their jerseys show that they are ineligible (any number in the 50s, 60s or 70s on offense). If the ball is tipped by an eligible receiver, then offensive linemen are allowed to catch the ball. If an offensive lineman lines up in a tight end position, he must report to the referees as an eligible receiver if he wishes to catch the ball.