It means simple and enjoyable - very easy to do. I comes from the literal expression in English, "It is as easy as eating pie." It was probably just a shortening of that sentence that created the phrase, and the public's desire to be straight to the point or quick and direct in speech. But because making it terse or quip (short in this case) by cutting out the verb from the sentence (and turning it into a colloquial phrase as a result), it became too short. It is thus now idiomatic - meaning that the words don't literally add up to mean together what the dictionary says each word taken separately would sum up to mean. Making it short, cute, and direct resulted in it being cryptic in meaning. Cutting out part of the expression made it difficult to understand without the typical native English social experience of "eating" a smooth (creamy) piece of pie.