When native speakers say something is “a piece of cake,” they mean that it’s very easy. It’s really common when we’re talking about things such as tasks, or tests (sometimes!), or games/sports. We often say it very quickly, as if it’s one word: “piece-a-cake.”
Another funny way to say something is easy is to say it’s “easy-peasy.” Kids even say, “easy-peasy, lemon-squeezy.” This doesn’t mean anything, it’s just fun to say.
Tim: What did you think of the history test, Susan.
Susan: Hmm, well you know history isn’t my strong point… How about you?
Susan: Oh no, don’t say that! You’re just making me feel worse.
Tim: Sorry to upset you, but it really was easy-peasy!
Susan: I’m gonna stop talking to you now…
As you can see, Susan is irritated by Tim saying it was an easy test — but if they’re friends and the test wasn’t important, she’s probably pretending to be annoyed.
Yuko: It’s my big tennis match next Saturday.
John: Yeah, wow, it’s great that you got into the semi-finals.
Yuko: To be honest, I’m feeling a bit nervous about it.
John: Oh, don’t worry, you’re an amazing player. It’ll be a piece-a-cake for you!
Yuko: Thanks, John. I hope you’re right.