As the name implies, an ingrown toenail (onychocryptosis) means that your toenail grows into your skin.
Typically, your toenail is supposed to grow straight outwards in length. In some cases though, the corners of your toenail break through your skin along the edges of your nail bed, causing inflammation.
If left untreated, your toenail can continue cutting into your skin, worsening the wound as your toenail grows. Without proper treatment, this can lead to severe pain and possibly infection.