Ham is a type of pork, or meat that comes from a pig. If you order a ham sandwich at a deli, you'll get a pile of thinly-sliced meat between two slices of bread.
Most ham is preserved in some way, either smoked or salted, and traditionally this meat comes from a hog's hind legs. For many people who celebrate Easter, ham is a typical main dish. Ham, defined in the 1630s as "meat from the hind leg of a pig," comes from the Old English hamm, "hollow or bend of the knee." If you're a performer described as a ham, it means your acting is way too theatrical or exaggerated.