A PIECE OF CAKE is a popular phrase that originated in America, in the southern states. It is believed that it was invented by slaves in the 1870's when a piece of cake was given to them by their owners as a prize for winning competitions like dancing. They thought it was so easy to win that "a piece of cake" ended up meaning "something very easy to do".
So, next time you want to say that something is not complicated or it doesn't need much effort, say it's a piece of cake!