The knit cast on is an easy way to get stitches on the needle so you can begin to knit. It's one of the first cast on methods I teach because it basically teaches you how to make the knit stitch at the same time that you're learning to cast on.
This cast on isn't just useful for beginners, though; it's also a great choice when you're casting on a lot of stitches and don't want to risk having too short of a long tail, or when you're adding stitches in the middle of a row.
Like most cast ons you will encounter, the knit cast on begins with a slip knot.
To make a slip knot, hold the yarn so that the tail of the yarn ball is hanging in front of your hand.
Wrap the yarn loosely behind your first two fingers of your left hand. The yarn should loop all the way around your fingers clockwise.
Take the part of the yarn that's attached to the ball and slip it back under the loop behind your fingers (shown here without a hand involved).
Then slide this off your fingers, pull to tighten slightly, and slip it over the needle.