It’s behind the second-last defender. The last defender is, usually, the goalie but it really doesn’t matter for the call. You can also not be off-side in your own half, or after directly receiving the ball after a throw-in, goal kick or corner kick or if the ball originated from either an opposing player or from a team mate that would already be considered off-side should they receive a ball (as in your teammate defeated the defenders in regular play with the ball). Furthermore, off-side is never applied to any backward pass, it must be from a pass toward the enemy goal.