Korra, from the Southern Water Tribe, is the newest Avatar, the one who can Bend all four elements: Air, Water, Earth and Fire in The Legend of Korra.
See, the Avatar reincarnates through the centuries. Each time an Avatar dies, they are reborn into the next nation, in a specific pattern: Air, then Water, then Earth, then Fire. This successive cycle mirrors the order of the seasons. The four Avatars before Aang were, in receding order: Roku, a male from the Fire Nation; Kyoshi, a female from the Earth Nation; Kuruk, a male from the Water Nation, and Yangchen, a female from the Air Nation.
Before Korra, there was Avatar Aang, the last Airbender. When we last saw him in Avatar: The Last Airbender, he was a 12 year-old boy who had just defeated Fire Lord Ozai.
The Legend of Korra picks up 70 years later, after Aang's death. We learn he and Katara had children, including Airbending Tenzin, who is selected to train Korra. But what else happened in between? How did he die?
According to Nickelodeon, after heroically ending the Hundred Year War, Avatar Aang and Fire Lord Zuko (Lord Ozai's son) worked together to restore peace and balance among the four nations. They transformed the Fire Nation colonies into the United Republic of Nations, a society where Benders and Non-benders from all over the world could live and thrive together in peace and harmony. They named the capital of this great land Republic City. Technology advanced at an exponential rate. (Even though the cars, music and radios are from our 1920s.)
Aang and Katara married and had three children: Bumi, a Non-bender; Kya, a Waterbender, and Tenzin, an Airbender. Aang trained Tenzin in Airbending and passed down to him Air Nomad teachings and culture. A following of Air Acolytes grew around them. They rebuilt the Air Temples and established new ones in the United Republic. The Non-bending Acolytes uphold Air Nomad teachings and spread peace and harmony through the world.
But Aang's 100 years in the iceberg caught up with him when he was in his 60s. His health began to fail. With the help of the Order of the White Lotus, Aang established safeguards so that his next incarnation would be protected from anyone who might do the young Avatar harm. At the age of 66, Avatar Aang passed away.
The fictional Republic City pays honor to Aang with an enormous statue on Aang Memorial Island. That island is also a way the cartoon creators and fans can pay homage to a hero who meant so much to so many. We see him in every Korra opening, and will not forget him.