Garter tabs are used to knit shawls when you want your garter stitch edge to be continuous. You start at the center of the shawl and you knit the garter stitch edge in both directions.
Using the method in this video, I used the Judy's Magic Cast On then flipped my work over so that my purl bumps were showing, then began to knit flat on one side back and forth to create the garter stitch edge.
Once I had knit the 7 rows, I turn my work and pick up 3 stitches, 1 stitch on each garter ridge that I've created. Then I knit my last 3 stitches from the other side of the Judy's Magic Cast on and my Garter Tab is now complete and I can continue my shawl on both sides with a nice clean edge.
Learn more about the Golden Hour Shawl here.