The peel and crush washers are used to properly index the muzzle device (Flash Hider, Compensator, etc.)
The Peel Washer is a washer that splits to modify the thickness of the washer to allow you to index or align the muzzle device properly on the barrel.
The Crush Washer is a washer that will flatten as pressure is applied allowing you to index or properly align the muzzle device on the barrel.
(Either option works, and you only need one or the other. I prefer the crush washer because you put it on once and it is done, the peel washer may involve putting it on then taking it off to split it for proper alignment. It is also believed that these should only be used once, if you change your muzzle or barrel it is said that it is best to get a new washer.)