The easiest way to describe a high wrist hold is, if you hold your arm out with thumb up, let your hand drop down. You will then have nearly a straight line across the top or your arm to your index finger. Contact with the bow is primarily in the web between your thumb and index finger.
A low wrist hold is best described as just holding your arm out and making a fist. Contact with the bow will be fairly even across your palm.
Most recurves are best shot with a high wrist hold. This is generally insured by the shape of the riser.
Longbows have a straighter grip (some are indeed straight), and shoot best with a low wrist hold. Some archers prefer a longbow style grip on recurves and use a low wrist hold. Conversely, some modern longbows have a recurve style pistol grip and force a high wrist hold.
Shoot whatever works best for you. I shoot high wrist with my recurves and low wrist with my longbow. The long bow doesn't like a high wrist hold.