Selvage stitch

I am knew at knitting and there is a beginner baby patter I would like to try. Though i am not sure on how to make a selvage stitch. Can someone explain or give me the how too?

Selvedge stitch comes from the words ‘self edge.’ Just add one stitch to each edge. Work in stockinette–knit on the right side, purl on the wrong. They’re added to make for easy seaming.

Thank you I just did not know what that meant. I truly appreciate your help :XX: