Strip is a call that was removed from the rules a while ago, however, it's still mentioned in the rules and making the call is habit for many players. Strip describe a specific type of foul. The call is 'foul', the receiver is then asserting that they had control of the disc before the foul caused control to be lost. In most places on the field, the distinction would not matter. In the scoring end-zone however, what would have happened had the foul not occurred is a point. A regular receiving foul in the end-zone that is not contested would have the receiver tap the disc in at the front of the end-zone.
So how does this scenario play out: Offensive receiver catches the disc and is about to land in the end-zone for a score. Contact from a defender with either the disc, or the receiver, causes them to drop the disc (or it's ripped from their hand). Receiver calls 'foul"....
The defender can 'contest' the foul, claiming that the contact wasn't caused by them or was incidental - here the disc is returned to the thrower.
The defender can agree that they committed a foul, 'no contest'.
The receiver is also saying the pass was complete and the outcome should be a point. It is possible for the defender to say the pass was incomplete (despite fouling them) and the outcome would be treated as an ordinary 'non-strip' foul. The result would be the receiver gets the disc at the front of the end-zone. If the defender agrees that the receiver caught the disc, and therefore the foul was a strip - it's a point.