I like how confusing it can be to read about simple.
Or rather, how complex a subject it is to discuss :)
By definition simple is made of a single element (from latin simplex).
There are quotes coming to my mind, like 'less is more', 'perfection is when there is noting left to be removed'.
It's a lot about mental model: to be able to manage a project one has to be able to keep it all in one's head, thus simplifying is mastering chaos
It often is multi-layer though.
It's a loss less mental model compression
An example that I have in mind is monotheism evolving from other older models, it's simple in a sense, and also more portable across different cultures, but it's deep (no proselytism intended).
PS: cannot find where 'simplex' comes from :(