Hi, and welcome to KH forum.
When you knit the first 6 rows you will be getting garter stitch. That is a nice stitch that doesn't curl and will keep your edge flat. It looks the same on both sides and is bumpy in rows or ridges. RS means that as you work row 7 the side that faces you will from here out be designated as the right side, the side that will face out as the item is worn or whatever. You might like to put a little safety pin on that side as you work that row, or pull a bit of yarn through and tie a bow on that side or something to mark it.
All the time you knit those first 6 rows you knit the row and then exchanged the needles in your hands and knit back again. You do the same thing here, and actually nothing will be different but they are going to begin here to make a stockinette stitch appear on the RS.
After you knit row 7 you exchange the needles in your hands so you can work back across the stitches, but on row 8 you will purl the row. That makes a bumpy side on that side and the other side will now have 2 smooth rows. That is the st st (stockinette stitch), that it tells you to continue doing. So from here on, until the piece measures 10 inches you will be alternating knit and purl rows.
Rows 1 to 6 you knit each row, row 7 you also knit and row 8 you purl. The next rows will be: row 9-Knit; row 10-purl; row 11- knit; row 12- purl, etc. All odd numbered rows will be knit and all even numbered rows will be purled. But the best way to think of it is that when you have the needle in your left hand ready to begin the next row, if the stitches on the top of the work (when the piece you are developing is held so that it is flattened out between the two needles), are the smooth side (with the little vees) you are ready to knit that row. And if the all bumpy side is facing up you should purl that row.
I hope that helps.