Stitch markers are just like bookmarks. They go on your needle between stitches so you know easily when to do whatever it is you have to do--increase, decrease, change stitches, etc. They just help you keep track of things. You can even just use a short strand of yarn tied around your needle instead of a purchased stitch marker. When you get to a marker on your needle, you just slip it from one to the other.
Once in a rare, rare while, a pattern will tell you to mark a particular stitch--some markers are designed to open and close just for this option, though a strand of yarn tied in the stitch just as well.