homoiconicity
1. Lisp S-expressions
An S-expression in Lisp is a list where the first element is (always?) a function
S-expressions can be evaluated to call the function using the rest of the list as arguments.
(+ 2 3)
The arguments can themselves be S-expressions, which will be recursively evalutated. Numbers are "self-evaluating", and always return themselves. Variables will return their value they are bound to
(+ (* 3 4) (* 2 1))
But we can also use the quote operator to prevent the S-expression from being evaluated. Then, it is treated like a plain old list that we can perform our usual list operations on
(cdr (quote (+ 2 3)))
This is what homoiconicity means. You can pass this list around and even modify it (although I'm told that is bad style). Any list that represents a valid s expression can be evaluated:
(eval (quote (+ 2 3)))
eval will first evaluate the (quote ...) S expression to yield the list. It will then evaluate again on the resulting list.
1.1. example
The following gives an expression when evaluated:
(mapcar 'car '((+ 2 3) (2 1) (3 3)))
which can then itself be evaluated:
(eval (mapcar 'car '((+ 2 3) (2 1) (3 3))))