The phrase "piece of cake" means that something is very easy to do or accomplish. It's often used in a sarcastic way when someone has accomplished something difficult, as if it was no challenge at all.
My diet was good 98 per cent of the time, so one chocolate or occasional piece of cake for my son's birthday wasn't going to ruin everything. (womenshealthandfitness.com.au)
A gluten - free diet was already difficult; but now it seems like piece of cake compared to cooking egg - free, dairy - free and soy - free. (dishbydish.net)
100 % — I'm the person who's eating it from the tub with a spoon and asking for the corner piece of cake because it has more sweet, creamy buttercream goodness. (justsotasty.com)