This is a 15-yard penalty which is called when a player blocks another player below the knees. Any block below the knees, whether from in front or behind, is illegal, mostly because it is extremely dangerous.
A low block is illegal and dangerous because it has the potential to seriously injure the knees and lower legs of the player being blocked. There have been numerous cases of illegal low blocks causing serious knee injuries. This type of illegal block is sometimes known as a cut block. A cut block is when an offensive player on the line of scrimmage attempts to get the opposing player on the ground by diving at his lower extremities rather than blocking them upright. Many players see this play as being on the fine line between a dirty play and a legal play. As a result, many players do not block this way often.