It is a sensor that senses what is commonly referred to as spark knock.
Now, let'* get serious and be a little more descriptive for all that might have a slight interest in helping you resolve your problem, of which I am sure there are a few.
First, what exactly do you own? Model, year, and trim level if you please. It also helps for future posts and requests, if you put that info in your signature line.
Second, we can certainly spend a bunch of time discussing Knock Sensors and what they do, or we can allow you to get real descriptive and tell us exactly what symptoms your car is exhibiting.
Mike: 1997 SE w/ 183K, aka White Horse
, Gutted Airbox w/K&N, drilled 180* t-stat, tint all around, clear corners w/SilverStars, "PONTIAC" light up tail, PEMs, NGK TR55s