if is a special form, which means that it is evaluated in a special way (the usual way is: evaluate all the arguments first, then call the function).
In the if special form, the test is evaluated first; then only one of the thenform and elseform is evaluated. This allows us to avoid errors that could occur otherwise.
(if (jester-food? x)
(write "Yecch!")
(eat x))
We don't want to eat before testing; the evaluation of if as a special form ensures this.
The function write is called for its side effect (printing) rather than its value.