A pinned stock is an M4-style adjustable stock that has been pinned in a fixed position. It looks and feels like an M4 stock but doesn't adjust. It would probably be a felony in your state to alter it to an adjustable configuration. It might be possible to pin it in a shorter position that suits you, though.
If you want a shorter stock and you live in one of the few states that restricts adjustable stocks, you can buy fixed stocks of various lengths; the shortest fixed stocks are the same length as an M4 adjustable stock collapsed to the shortest position.