In the NFL, only certain players (receivers, backs, etc.) are allowed to catch the ball on every play. However, if the offense wants to allow a player who is usually prohibited from catching to catch the ball, they may do so. The player in question must notify the officials of their intent to be an eligible receiver, and the official will tell everyone on the field. If the player does not notify the official, but lines up in an eligible receiver position, a 5-yard penalty will be assessed on the offense.
The player who reports to the officials as being eligible to catch a pass does not actually have to catch a pass. By informing the official, they are only making themselves eligible. Their intent may not even be to catch the ball, but rather to confuse the defense, or run down field to block for a receiver or other player making a catch.