I checked in the Glossary, just in case I was remembering wrong, and it says SS means slip stitch. Five rows of just slipping stitches makes no sense, which is why I checked. Is there something, somewhere in your pattern that tells you what it means? Sometimes things like that are well hidden. What is it you're making, can you link to the pattern or at least tell us a name?
Cheating is an option. . . . Cheaters never win and winners never cheat, but smart knitters who want to retain an iota of sanity do, cheerfully. ~~Kory Stamper
Agree w/stocking stitch (stockinette). It's Rows 4 through 8 (five rows) of each leg, and they're stocking stitch, not slipped stitches. This is an unorthodox abbreviation, but looking at the clear photo and the designer's instructions helps in figuring it out.
(I think "st st" would have helped even more, though....)