Literally: as simple as hello.
Meaning: this expression is used for anything that is very easy and comes naturally.
it's as easy as shelling peas; easy peasy.
Origins: 19th century, at a time when to say just a simple hello was perceived as much easier, uncomplicated, versus today where you have a handshake and “the bises”.
Examples: commencer est simple comme bonjour!
Getting started is easy!