This is a philosophy. One which many people that write Ruby subscribe to. The fundamental idea is: create a DSL that makes it very easy to implement your application. It is what made Rails different when it was created: it is a DSL that makes expressing web applications easy.
I don't know its history well enough, but it seems to originate from Lisp. PG wrote about it before [1].
It can result in code that is extremely easy to read and reason about. It can also be incredibly messy. I have seen lots of examples of both over the years.
It is the polar opposite of Go's philosophy (be explicit & favour predictability across all codebases over expressiveness).