A ball that lands in your court first may then rebound off your own wall and stays in play. Only a ball that reaches the wall without touching the ground first is a fault.
The wall is why padel feels nothing like tennis: what would be a lost ball there is your second chance here. Volleys, serves, groundstrokes and back-wall shots together account for around two thirds of all actions — so wall balls are routine on the 20 by 10 metre court, not an exception. The classic beginner error is running away from the ball: you end up rushed and make contact behind your body. Instead, let it bounce off the floor first, then off the glass, and play it under control in front of you. To do that, stand about a metre off the back glass when you are defending, otherwise the rebound jumps into your back. One thing that gets missed: the bounce depends on the material. Glass gives faster, longer rebounds than concrete, and even the brand of ball noticeably changes length and speed.