At that point in the scene Robin (who has been instructed by Mistress Page to enter at this moment) interrupts the fat lover and announces that Mistress Page has arrived and that she is sweating and out of breath. Falstaff does not want Mistress Page to know that he is wooing Mistress Ford, so he hides behind an arras (a tapestry or screen).
Mistress Ford enters and proclaims that Master Ford and several officers are coming to the house at that moment to apprehend a man whom they suspect is with Mistress Ford. Ironically, Mistress Page does not know that Ford is really coming. Her lines have been rehearsed: they are part of the prank. Her lines are intended to frighten Falstaff, and their scheme is working. Mistress Page suggests that perhaps they can hide the offender inside the buck- basket (laundry basket), but Mistress Ford comments that the man is too big to hide in it. The terrified Falstaff, however, runs out and forces himself to hide in the basket.