"Next round: [K2, P2] for 20 sts, place marker. Repeat around. 6 stitch markers total."
A marker is a plastic ring, or a piece of yarn tied into a circle. You can use anything to mark your stitch. Some people have even cut drinking straws into small rings and used them, others have used paper clips, just about anything will do. And of course you can buy stitch markers at stores like Wal Mart, JoAnn's and Michael's etc.
So what you are going to do is to (K2, P2) for 20 sts. Take your marker and slip it on to the needle after your last P2 stitch. Then repeat again (K2, P2) for 20 stitches and place another marker on your needle. You will continue doing this till you have a total of 6 stitch markers on your needle.
Stitch markers make it easier to find "that" stitch you will be needing in the next row. It keeps you from counting because a lot of times the directions will tell you to knit to the next marker. I hope this helps.