SSP is the abbreviation for slip, slip, purl. This is a single, left leaning decrease, and is the purl version of the SSK.
How to work a SSP
With yarn in front and the right needle behind the left needle, twist the right needle so it can go into 2 stitches together from left to right. Slip 2 stitches to right needle. Put left needle into back of slipped stitches and p2tog.