K1 P1 Ribbing - Knit 1 Purl 1
Knit one stitch, pull your yarn forward, purl the next stitch, pull your yarn back. Repeat. Fairly simple and can be a bit time consuming when you're first learning how to knit as a beginner.
This combo of stitches makes things have a good deal of stretch and springiness. It's great for a cuff on sleeves, necks, sock tops and waistbands. And it doesn't always need to be K1 P1. It can be in any combo of these knitting stitches together like K2 P2 or K2 P1.