When you say something is a "piece of cake," it means it is very easy to do.
In the 19th and early 20th centuries, African Americans started "cake walks." They walked or danced around a cake in pairs, and the best couple won the cake. This is how the idioms "cake walk" and "piece of cake" came about -- both meaning that something is very easy to do.
A: Hey! How did you do on the exam?
B: I got an A. The exam was a piece of cake.
12 step recovery jargon referring to someone new who talks about how great life is, now that they'resober. Usually meaning that the person is out of touch with reality.
The new guy seems pretty happy for a dude who has no job, no money and no family. He must be on a pink cloud.