Knitting sleeves on circular neeles

I’ve never done this before. Directions say to cast on 38 sts, place end of rnd marker, join in rnd. What does this mean?

Look at this page for a couple ways to knit in the round, including joining, which can be as easy as just knitting the stitch on the other side of the needle. This page also has some pictures to help you understand it.

These are the very common directions for starting a piece of work to knit in the round.

If you have not knitted in the round before watch the videos here.