Yeah, we've all got one.
Behind that bolt is a spring and a few other little bits and pieces internally that operate (with hydraulic pressure from the oil pump), a tensioner which keeps the camchain tight. The camchain runs from the big sprocket on the end of your camshaft (behind the big round cover on the side of the head) down to a little sprocket with half the no. teeth of the cam sprocket which runs off the crank.
When you want to release pressure on the cam chain for removal or fitting of the head, you can release the oil and spring pressure with this bolt.