The cylinder head is an engine component that sits on top of the cylinder block.
Cylinder heads have a threaded hole in them, into which spark plugs thread. The electrode end of the spark plug protrudes into the depression in the back side of the cylinder head (the side that seats against the cylinder to create the combustion chamber). These depressions can be shaped differently, and the angle at which the spark plug enters them also varies.
When the piston is at TDC, the volume of the cylinder is compressed into the space that exists between the depression in the cylinder head and the crown of the piston. Just before the piston gets to TDC, the spark plug ignites the fuel:air mixture, and the combustion of this compressed mixture exerts force on the piston, causing it to move back down in the cylinder.
Often, a gasket is places in between the cylinder and the cylinder head to insure a good seal. However, if the mating surfaces of the cylinder head and the top of the cylinder are perfectly flat and smooth, it is often possible to run without a head gasket, increasing compression. The manufacturers of Sachs mopeds eliminated this issue by creating a top end in which the cylinder and cylinder head are cast as one part. However, the downside to this manufacturing choice is that one cannot remove the cylinder head to see how/if the piston is moving in the cylinder or to access the top end for removing carbon deposits.