Extremely easy; a piece of cake — used idiomatically (e.g., 'it's as easy as breakfast').
その作業は朝飯前だよ、すぐ終わる。
That task is a piece of cake; it'll finish quickly.
彼にとってはこの問題は朝飯前のことだ。
For him, this problem is child's play.
こんな簡単な仕事が朝飯前に思えるなら自信がある証拠だ。
If you find this simple job like a piece of cake, that's proof of your confidence.
Literally 'before breakfast' — the original, literal meaning.
朝飯前に散歩をする習慣がある。
I have a habit of taking a walk before breakfast.