This is SSK, which is a decrease
sl 2tog knitwise, k1, p2sso means to slip two stitches at one time as if you were knitting them together, then knit 1 stitch and pass the two over the one you just knit and off the needle.
The YO stitches in the row will offset the decreases, so you should come out even. When the instructions call for a YO, bring the yarn to the front between the needles and then knit the next stitches as called for in the pattern. This will create an extra stitch and an eyelet.
YO - http://www.knittinghelp.com/apps/flash/video_player/play/109/1