Fungus. Is the most likely cause.
Injury or fungus. You may have injured the toenail or you could have a fungus.
Toenail line. You probably had some trauma in the past to the growth plate which nows constructs the toenail with the line. This might be unsightly, but unless thick and uncomfortable, can be ignored. It might be an excuse to get a pedicure with polish if that fits your lifestyle.