What does this mean?

General Knitting
1

Work in stockinette (knit 1 row, purl 1 row) until piece measures 5-1/4" Mark each side (this will be the marking for the sleeves) Continue working until piece measures 10" or desired length.

What does it mean to Mark each side?

Thanks!

2

Tie a little piece of yarn, or put a pin at the edges. When you go to put the sleeves on, this is where you’ll line up the edge of the sleeve.

3

Thanks very much!