Hamlet is alluding to a story (familiar to Shakespeare’s audience) in which an ape takes a basket of birds onto the top of a house, opens the basket, and watches them fly away. Attempting to imitate them, he climbs into the basket and jumps out, only to fall off the roof and break his neck. Once again Hamlet is speaking ironically, telling Gertrude to go ahead and reveal his plans to Claudius which would be akin to the ape’s action (letting the cat out of the bag, so to speak) and would likely have the same fatal consequences.