In a simple way, is like a spring that minize the impact in your bit and your string. Its usually used on hard environments whit high chances of axial vibration, the shock sub absorbs part of this axial vibration and minimize the wear in the bit (e.g usually avoid broken teeths/cutters). But not only "protect" the bit, also downhole tools such mud motors/RSS, mwd/lwd improve his life when you includes a shock sub in the BHA.