"PSSO" stands for "pass slipped stitch over." Knitters use this stitch when making decreases in a row of knitting, and it gives a left-slanting decrease.
"PSSO" is usually found as part of a fuller set of instructions, such as "S1, K1, PSSO." The first part of the instruction is to slip the next stitch from the left hand to the right hand needle without knitting it. Knit the following stitch and then insert the left hand needle into the front of the slipped stitch. Bring it over the top of the knitted stitch and off of the end of the right-hand needle.