Casting on is how you create the first row of stitches on a needle to begin a knitting project. The long-tail cast-on is the most popular method, giving a neat, elastic edge. Every knit project starts by casting on the required number of stitches.
Different cast-ons suit different needs — a stretchy cast-on for sock cuffs, a provisional cast-on when you'll pick up the edge later. The long-tail method is the reliable all-purpose choice most knitters learn first.