What it is

A forward pass is a pass thrown toward the opponent’s end of the field. Each play may have only one forward pass, and it must be thrown from behind the line of scrimmage.

In a game

Only eligible receivers can catch one. If it hits the ground without being caught, the play is an incomplete pass and the clock stops.

More detail

A pass thrown sideways or backward is a lateral, and it is treated differently: it can be thrown at any time and by any player, and if it hits the ground, it is a live ball.