Avatar Temple

The Avatar Temples are shrines for the Avatar. They are first shown in The Southern Air Temple, when Aang triggers the Avatar State and the statues in all of the temples start to glow, letting people all over the world know that the Avatar was back.

So far, it has been shown that every nation has at least one Avatar Temple. During the Winter Solstice, Avatar Roku destroyed the Fire Sages Temple when he was being channeled by Aang. Also in the Earth Kingdom shrine you can see a someone meditating, so the temples could also have been a quiet place for concentration.

It is unknown why the creators don't show the temples glowing when Aang first enters the Avatar State after he is freed from the iceberg (while he is fighting Zuko on his warship.) Perhaps it was because Aang entered it by physical trauma i.e. he falls in the water, and not by meditation, concentration, or other emotional or non-physical triggers.