what is it?

It refers to the bolt/bolt carrier of a high cyclic rate AR unlocking slightly after the front of the bolt carrier smacks the rear of the barrel extension, especially in full-auto.  Thus the AR buffer has a number of moving/padded weights that "follow home" the major recoiling mass and act as a "dead blow" no-bounce hammers during that period while the hammer is swinging up towards the firing pin.
Carbines unlocking at higher that normal (read: rifle) pressure are particularly prone to this, thus several "heavy" buffers are out there that serve as more efficient dead blow/no bounce hammers in an effort to increase reliability via a part that is not identical to the parent rifle system, i.e., the Carbine length buffer.

so what happens when it opens slightly...are you saying that there will be a misfire? i understand what you are saying but is the end result of a bolt bounce....mis feeding....light primer strikes, failure to eject?
thanks!

Misfire.
Misfire with a light strike.

Usually a clearable malfunction via Immediate Action (IA).

Bacially your weapon stops firing on full-auto with the bolt closed.  You are sure rounds remain inthe magazine.
Pull trigger again and nothing happens (not even a hammer drop, as that has occured already).
Execute IA.

On semi, you pull the trigger and sense the hammer falling but no ignition.
Pull trigger again and nothing happens (not even a hammer drop, as that has occured already, but you are confused and pull the trigger again on instinct).
Execute IA.