also: split end · flanker · slot · X · Z · the three-receiver names
The older naming system for receivers, based on where they stand and whether they are on the line. It survives inside modern position letters, which is why X and Z are not interchangeable.
3 meanings, all below. Jump to the one you heard.
A receiver split out wide who is ON the line of scrimmage. Being on the line is the definition and it carries real consequences: he is bound by the rule requiring seven players on the line, he cannot be in motion at the snap the way a backfield player can, and he is the man a cornerback can press without help.
He is usually the X in letter notation, and "X" carries this on-the-line meaning rather than being an arbitrary label.
X, the X receiverflanker, slotA receiver split out wide who is OFF the line of scrimmage, standing a yard or so back. That single yard is the entire distinction and it buys two things: he can go in motion, and he gets a cleaner release because a defender pressing him has to account for the extra space.
He is usually the Z. A coach putting his best receiver at Z rather than X is normally doing it to protect him from press coverage, which is a real coaching decision hiding inside a letter.
Z, the Z receiversplit end, slotA receiver aligned inside another receiver rather than out by the sideline. Slot describes position relative to a teammate rather than distance from the formation, so a slot receiver can be almost anywhere between the tackle and the numbers.
What changes is who covers him and from where. A slot receiver is normally matched by a nickel or an overhang rather than a boundary corner, he has grass on both sides of him rather than a sideline to be pinned against, and he is inside the leverage of any deep help.
inside receiversplit and nasty split, bunch and stackThe old three-name system is often described as obsolete and it is not: it survives inside the letters. X, Y, Z and H are not arbitrary labels, they encode on-the-line-ness and formation side, which is exactly what split end and flanker named. A coach who learns the letters without learning what they came from will eventually be surprised by an illegal formation.