If you're knitting flat you knit all rows. If you're knitting in the round you knit a row, then purl a row to get one garter ridge.
It helps when asking questions to tell us what you're making and give a link to the pattern if at all possible. The answer may vary depending that info. Without knowing what you're doing I'd say that they want you to knit 6 rows (3 ridges).
When asking questions ALWAYS post the name and a link for the pattern if you have it.