A decrease, in knitting, is any method that decreases the stitch count! Decreases are used to shape the fabric; at sleeves, at shoulders, at the top of hats, etc. Decreases are also used within lace and other stitch patterns, to manipulate the shape of the fabric itself.
This post describes how to work a sl1-k2tog-psso. Sl1-k2tog-psso (slip 1, knit 2 together, pass slipped stitch over) is a double decrease, meaning it decreases 2 stitches from the total stitch count.
Step-by-step instructions on the ssk stitch or ‘slip slip knit’
Instructions for knitting 2 stitches together.
Instructions on how to purl 2 stitches together.
Examining Knit Decreases: K2tog, SSK, and Sl2-K1,P2sso