People who are rangy are tall and thin, with long arms and legs. You could describe a skinny young basketball player as rangy.
This adjective most often describes someone who's very tall and slim, or even a little gawky, like a teenager with big feet and long legs who hasn't finished growing and is awkward in their own body. More rarely, rangy means "tending, allowed, or allowing to roam," like a rangy stray dog or a wide, rangy field. It comes from the verb range, or "move freely over a wide area."