This is a decrease most commonly seen in older patterns.
1. Slip 1 stitch knitwise from the left needle to the right needle.
2. Knit the next stitch on the left needle.
3. Insert the tip of the left 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:
1. Purl 1 stitch.
2. Slip the next stitch knitwise to the right needle, and return it in this changed direction to the left needle.
3. Transfer the purled stitch (from Step 1) from the right needle to the left needle.
4. Insert the right needle into the slipped stitch and bring it over the purled stitch and off the needle.
For knitting decrease on purl row you can meet such symbol: 