I know this gets confusing when you're first starting out! My eyes were crossing the first time I tackled it but I finally got it and now it's easy, so just hang in there!
What you're doing when knitting "in the round" is knitting a tube. It has no seam, it's just a simple tube. Look at a pair of store bought socks and you'll get the idea.
To get that tube effect you'll want to join up the last stitch you cast on to the first stitch you cast on. That closes up the circle at the top and gives you the first row of the tube. When you do this, make sure that those little nubs at the bottom of your cast on are all facing down and haven't twisted around the needle. If they get twisted you'll have to rip back and start again.
You place a marker there because you're going to have to shift some stitches around later in the pattern to construct the heel section. According to your pattern, the marker lets you know that's the center of the heel stitches. This will keep you oriented as you later re-divide your stitches to work the heel part.
But first you're going to connect up those first and last cast on stitches. You can do this in a couple of ways but probably the simplest is to just start your knitting! For a couple of rows it'll still be separated but as you go it'll close up. Then you can use the tail of yarn that's hanging there to sew those couple of rows closed when you're all finished.
Amy has some great videos on various stages of sock knitting and Silver's tutorial is excellent, too! You might want to check those out so you have a clearer picture of how all that yarn magically turns into socks!
Good luck. Let us know how it's going and if you have any other questions!