You just posted your question, so maybe you are still in the Forum.
Stitch markers are used to keep track of sections of complicated patterning on the row.
I just posted a photo of my aran afghan progress, click the link below, and you will see that I have little orange stitch markers on the cable. See how they mark certain sequences of patterning?
Stitch Marker placements
and scroll down to bottom of the page.
Stitch markers are handy for keeping track of many things on the row.
When a pattern WANTS YOU TO put a marker, it will use this abbreviation: PM for PLACE MARKER. Or they will outright say it.
Every time you come to the marker, just slip it from the left needle to the right needle, and continue with the pattern stitches called for.