A cam can be either a removable, or built in, irregular shaped disk that when engaged creates a stitch pattern. Notice the irregular shape of those in the picture.
Cams be be plastic or metal, separate or in a stack, large or small as in the picture.
Modern machines use electronics or computers to do the stitch patterns, but the older vintage ones used the cams with mechanical linkage to make the patterns. Cams went out of vogue in the late 70s to early 80s I believe.
Joe