Who/what is Klaatu in the movie "The Day the Earth Stood Still?"

Klaatu (played by Michael Rennie) arrives in a flying saucer in Washington, DC, accompanied by Gort (played by Lock Martin), as a peaceful ambassador from an extraterrestrial confederation; but when he presents a harmless device as "a gift for [the American] President…to study life on other planets", his intentions are misinterpreted by one of the soldiers that form his welcoming committee, who shoots him. To protect Klaatu, Gort destroys the surrounding artillery.

Klaatu is taken to a hospital, where he quickly recovers. On learning that the government does not expect other nations to be willing to meet him, he takes residence in a boarding house, using the surname "Carpenter", the name found on the clothes and suit case obtained by himself, and befriends Bobby Benson (played by Billy Gray), a boy who stays there with his mother, Helen Benson (played by Patricia Neal), and visits Professor Barnhardt, a brilliant scientist (played by Sam Jaffe). When Klaatu/Carpenter is brought later to him, Barnhardt agrees to summon scientists from all nations, and asks Klaatu for a show of power, thus exhibiting that the extraterrestrial alliance would threaten Earth if Earth threatens the peace in space. Klaatu, in reply, neutralizes electric power all over the world (with some notable exceptions, such as hospitals and airplanes in flight) for 30 minutes. Klaatu's action is taken as an act of war.