Slip 1: You just move the stitch over to the right needle without knitting it. It's usually best to slip purl wise, but pay attention to your pattern, it might ask you to slip knit wise.
k2tog: knit 2 stitches together. you'll put your needle into 2 stitches at once and knit like you would regularly. This is a common decrease, you're eliminating a stitch with this one. It's a right slanting decrease, btw.
psso: Pass Slipped Stitch Over. Here's an example: sl1, k1, psso. You slip a stitch, then knit a stitch. Then, you'll stick your left hand needle into the stitch you slipped that is now on the right hand needle. You'll pull that stitch over the knit stitch and off of the right hand needle, letting it drop. Now you've decreased again, by one stitch.
If you did sl1, k2tog, psso you're doing some very rapid decreasing, and just follow what I said above (replace the k1 above with k2tog). It looks a little complicated, but once you have a rythym down it's not so bad
Hope this helps...Denise
p.s. sl1, k1, psso is a left slanting decrease.