Failed businessman Simon Williams was approached by Baron Zemo, who offered him great ionic power in exchange for helping him destroy the Avengers. Simon became Wonder Man, and joined the Avengers by saving them from a staged attack by Zemo. But Simon became friends with the Avengers, and decided he could not betray them. Without Zemo's treatments keeping him alive, Wonder Man died. Until he woke up years later - because in comics, no one ever stays dead.
This was the first of Wonder Man's poor costume choices. While it has a tremendous amount of details making this costume busy, the real flaw in this costume is its red and green Christmas color scheme. It's like the entire costume is a big ugly Christmas sweater.