The simple (literal) meaning of “piece of cake” is this:  

But “piece of cake” is also a common idiom that means something is easy.

• I can do that.  It’s a piece of cake.
• Did you finish the job? >> Yes, it was a piece of cake.
• More examples on Youglish.com  Can you tell which of the examples use piece of cake as an idiom, and which are using the literal meaning?