What is a backwards pass? Backwards passes are not thrown forward--they are thrown backwards (hence, the name) or laterally. Unlike forward passes, there are no limits to the number of backwards passes in a play, so you can throw the ball backwards or laterally from player to player anywhere on the field. For an example, check out the video below of a crazy 8-lateral kickoff return for a touchdown that allowed Miami to beat Duke back in 2015.
The major risk of throwing a backwards pass is that if the ball touches the ground it is still "live" and is considered a fumble, so any player can recover the ball.
Can you mix backwards passes with a forward pass in a play? Yes, but the rules for forward passes still apply. That means you are still only allowed 1 forward pass per play, and the player throwing the forward pass must be behind the team's line of scrimmage.