The rating for the breaking strength of your line. While there are all kinds of variables (lures, water clarity, etc.), you should match your line to the fish you’re after. 4 lb test for panfish. 6-20 lb test for bass, walleye and pike. Heavier line for catfish and other large species.
Spools of fishing line are measured in yards (and in meters). Be sure to not overload the line capacity of your reel which can cause tangles.