Sewing - the part of the sewing machine that holds the fabric while the needle pierces it.
The presser foot can be raised and lowered with a small lever at the back of, or beside, the needle. When up, it allows free movement of the fabric. When down, it presses the cloth against a base plate. The base plate has a couple of textured moving parts (the feed dogs) that keep the material moving past the needle at an even rate. (You can sew with the presser foot up, but you tend to get very uneven stitches. This is because you have to move the fabric yourself, and keeping it moving at an even rate is almost impossible.)
There are dozens of specialised presser feet on the market. Most machines come with a standard presser foot (which most people use most of the time), a zipper foot (for installing zippers), and a buttonhole foot (guess what that's for?).