Stitch markers sit between two stitches and are "slipped" (that is they are moved from one needle to the other without being knit or purled) every row.
You use them to indicate something and the possibilities are many.
1. You may use them every pattern repeat across a row so you can be asured you have worked the pattern correctly.
2. You could use it to indicate the a point where you'll need to work an increase or decrease for say, a hat.
3. When knitting in the round, you would definitely need it to mark where a round begins/ends.
You get the point