Okay, to answer your question, huge copypaste from the Avatar Wiki:
- The Avatar State is a defense mechanism, in which an Avatar may freely access the knowledge and powers of all previous Avatars. This provides the Avatar with great powers, as well as mastery over all four elements (even if the Avatar has not yet learned all four bending disciplines). It was by entering the Avatar State that Aang was able to freeze himself and Appa into an iceberg for 100 years, by using a combination of Airbending and Waterbending. While in the Avatar State, the Avatar's eyes glow - as do their arrow tattoos in the case of Avatars from the Air Nomads, although if the body has received an injury deeper than the tattoo then that spot will not glow in the state. It is not known if re-tattooing the area (once the injury has healed) would enable it to glow again.