One penalty that you'll hear called at times in football games is the ineligible receiver downfield penalty. What is this rule and what is it all about?
In any offensive formation, there are 5 eligible receivers. Technically, the quarterback is also an eligible receiver, though we normally don't see him going out for passes very often, unless it's a trick play. If one of the 5 linemen heads across the neutral zone, and a forward pass is thrown downfield, this is a penalty. Once the ball is caught by an eligible receiver, then the linemen can head downfield to block.
This is one of the reasons linemen need to have specific jersey numbers. It makes it easier for the officiating crew to determine who is an eligible receiver and who is not, once the play is initiated. Here is a chart that explains more about the numbering system for football jerseys. Numbering System Chart