A stitch marker is a device for marking stitches in crochet or knitting.
Stitch markers can help crocheters and knitters keep track of any place in a design that might be time-consuming or challenging to find later, such as the beginning / ending of a round, or the beginning / ending of a pattern repeat. They're helpful to have on hand when crocheting socks, hats, and many other types of projects. They're also helpful to use when working with furry or hairy yarns such as eyelash yarn, where the texture of the yarn makes it difficult to see and count your stitches.
Pictured above: Clover locking stitch markers, which are designed especially for knitting and crocheting. You can compare prices on these stitch markers at Pricegrabber.