Can you please explain more?

Homoiconicity means program and data have identical representation. But after you Lisp for a while the boundary blurs entirely.

X-bar theory says that in all natural languages, Noun Phrases, Verb Phrases, and depending on who you read other types of phrases all have the same form; a single Head (which may be composite) and a Tail with zero or more elements. Never mind that they've just discovered S-expressions. The point is that in the natural language program:

    multiply the radius squared and pi

Both the data-like parts and the program-like parts share the same structure.