Lambda the Ultimate

Morning everyone. I have a question about the internals of Lisp (as a family) and I came here hoping someone may be able to answer my question.

I just recently watched the Rainer videos again, and I was completely amazing at the reflective, or maybe the proper word is introspective capabilities of Lisp, espicially while a program was running. So I finally decided that I wanted to know how this was possible, and why it was implemented in more languages to such a scale. What portions of Lisp contribute to this? Is there one specific feature, or is it more like a group of features?

Thank you everyone.

-M.