Explanation: “It’s a piece of cake” means that something is really easy.
Imagine a conversation between a teacher and a student: “What is 2 + 2 ?”
“Oh, that’s a piece of cake! It’s 4!”
A piece of cake (you take an entire cake and cut a piece out and serve it on a plate) can also be just a piece of cake. This joke is funny because it plays on the double meaning of “piece of cake.”