White meat is “fast twitch” muscle and is used for short powerful bursts of activity, like the breast muscles, which are used for flapping the wings

Dark meat is “slow twitch” muscle and is used for longer duration activities like walking, hence the legs and thighs are dark meat.

Dark meat contains more fat and can be cooked longer without becoming dry. White meat becomes dry and tough very quickly if overcooked.

This only really applies to birds, mammals are made of red meat, which is like a combination of both fast and slow twitch and can do both sort and long duration activities