Decades ago, “Breakfast of Champions” was a slogan for Wheaties, a brand of cereal. The idea was that Wheaties was very healthy, and would help you perform like an athletic champion. Nowadays, the expression is often used ironically, especially to talk about eating something very UNhealthy in the morning. Example: “Ah, whisky. The breakfast of champions.”

It used to be a "wheaties" cereal advertisement slogan. The idea was that sports champions would eat and endorse this cereal to start their days. They would rotate pictures of winning teams and elite athletes on the box cover every so often. This was most popular in the 90s and is rarely referenced today.

It's a book, by an unlikely sounding author. :) And apparantly a movie.

It describes a really good breakfast! People generally use it to describe a hearty, big breakfast that will give you lots of energy. It comes from an advertising slogan and it can be used ironically.