To knit in garter stitch simply means you knit every row when making a flat piece of fabric. (When knitting in the round, you would need to alternate between knit rows and purl rows in order to create garter stitch fabric.) This creates a kind of ripply fabric that’s the same on both sides. The following video demonstrates how to knit a flat piece in garter stitch:
How to Knit Garter Stitch
Cushion of Joy 