Meaning: 楽勝だ、とても簡単なこと、楽しいこと
For Example:
‣ That history test we had in class today was a piece of cake! I'm pretty sure I got A on it.
今日クラスであった歴史のテスト楽勝やったわ！絶対A（90点以上の良い成績）や。
‣ A: Do you think you can handle this project?
B: Sure. It's a piece of cake. Leave it to me.
A:このプロジェクト対処できるかい？
B:もちろんです。簡単ですよ。私に任せて下さい。
Synonym:
as easy as pie, a cakewalk