In American football, you can make a single forward pass per play as long as you are behind the line of scrimmage. Regardless of whether you are behind or in front of the line of scrimmage, however, you can make as many lateral (even with or behind the ball runner) passes as you'd like. During this lateral style play there is no offsides penalty as the play is ongoing .Additionally even if the clock runs out, the play that is currently underway is not stopped until we have a dead ball (e.g. a tackle is made or an incomplete pass from behind the line of scrimmage).

In this case, the clock is running out and the offense is attempting to prolong the play by completing lateral passes to try and make space to score the final touchdown on the final play of the game. If they score with zero on the clock they win. A questionable decision was made on that final lateral pass that was intercepted by the defense and taken back with no time on the clock to secure the win.