It's talking about lisp macros, idempotent languages, and a few other features of lispey languages. I'd suggest the book On Lisp, or Lisp in Small Pieces as good places to learn about it, but there are a ton of other resources that may be better suited to your needs.
And don't miss Sonja Keene's book "Object-Oriented Programming in Common Lisp" and Kiczales' "The Art of the Meta-Object Protocol". If you don't reach enlightenment after those, Libgen will refund your money.
