What is a "simple expression"?

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/14/sql-select.html
Description:

Hi there

To start off I'd like to say your documentation is great and very helpful.
Thanks for that!

https://www.postgresql.org/docs/14/sql-select.html in the section "WINDOW
clause" talks about "simple expression." I couldn't find any explaining of
what actually constitutes a simple expression, not by searching for the term
or checking the index.

Cheers,
Damian