Meaning/Usage: Describes something that is easy to do.
Explanation: In the 1870's, cakes were given as prizes for winning competition. To the winner, it was an easy task. Therefore, "cake" started representing something easy. Another phrase that is similar is "cake walk," which also represents something that is easy.
"That test we took was a piece of cake."
"I put the whole thing together myself. It was a piece of cake."
"I told them I had never done this before and they said it would be a piece of cake."
A. "So I heard you got an A on your math test."
B. "Yeah, it was a piece of cake for me."
Other Common Sentences
"It's very easy."
"You should have no problem."