Slip 1, Knit 1, Pass Slipped Stitch Over

Slip 1, Knit 1, Pass Slipped Stitch Over is a decrease, meaning you lose a stitch, and is abbreviated in many ways such as skp, skpsso, or slip 1, K1, PSSO.

It uses two stitches on your left needle and will result in 1 stitch on your right needle, thus losing or decreasing your number of stitches by one.