Psso refers to pass slipped stitch over, which makes a bound-off stitch in the middle of a row. Pass slipped stitch over is a decrease that appears in certain stitch patterns and in double decreases (decreasing 2 stitches at the same time). You can work it from the knit or purl side of your work.
Here’s how to work a psso on the knit side:
Slip 1 stitch knitwise from the LH needle to the RH needle.
Knit the next stitch on the LH needle.
Insert the tip of the LH needle into the slipped stitch and bring it over the knitted stitch and off the needle as if you were binding off.
To work a psso on the purl side, do the following:
Purl 1 stitch.
Slip the next stitch knitwise to the RH needle, and return it in this changed direction to the LH needle.
Transfer the purled stitch (from Step 1) from the RH needle to the LH needle.
Insert the RH needle into the slipped stitch and bring it over the purled stitch and off the needle.
To pass a stitch over means to bind it off.