Stitch markers are little round items, usually made of plastic or metal, that can be slipped onto the knitting needle to mark a certain place in a row.
Stitch markers are often used to mark the end of a row in circular knitting, where it is not obvious where the circle begins and ends. They can also be used to mark where increases, decreases or blocks of different stitches should be placed.
Stitch markers can be purchased at your local yarn store, or sometimes found objects can be used as stitch markers. The white plastic rings in the picture were in a container of blank CDs.
Handmade stitch markers are a beautiful gift for knitters and relatively easy to make if you have some beading skill.