A sear is a part in a gun that is activated by the pressure of your finger on the trigger. When the pressure of your finger on the trigger overcomes the friction resistance of the sear, the sear releases the hammer of the gun and it fires. The sear is designed as a friction release so that the shot is a surprise if the shooter is squeezing the trigger correctly, avoiding involuntary responses of the shooter in advance of the shot.
A grip, tab, or riser sear is intended to accomplish the same purpose in archery, although it doesn't actually do anything, like a gun sear does. It can be any raised surface where a thumbnail or fingernail can be pressed, and if it's pressed hard enough, will slip off. The shooter instructs his mind to fire when the nail slips off the sear. The signal to fire is intended to go directly from the thumbnail to the string fingers before the rest of the body has a chance to react, which is called a non-anticipatory psychotrigger.
An example is the post above. A nylon washer has been fastened to the riser in a place conveniently reachable by a fingernail. After the shot has been set up, the shooter presses his fingernail on the washer until it slips off, at which time he releases the shot.
This is part of a target panic avoidance system, more than just a way of avoiding involuntary physical reactions before the shot. It is beyond the scope of my response to your question to explain Joel's whole system, but it involves enhanced concentration within a closed loop system, and would be well worth your efforts to look into, whether you decide to adopt it or not.