If we would like to comment out a line of the code we simply use a semicolon at the beginning. To comment out a block of the code we use hash underscore #_ (reader macro).
;(1 2 3) #_(defn sum [a b] (+ a b))
If an entire line is dedicated to the comment we conventionally use a double semicolon. When a comment is in line with the code we just use a single semicolon. We could also put a comment inside of the function (Docstring), after its name but before parameters.
;; Our full line comment (defn sum "Returns sum of a and b" [a b] ; our inline comment (+ a b))
Finally we can use a comment macro (Rich Comment) for the code we are only going to use for development/debugging. For example we could have two versions of the same function but with different implementations. It returns nil when evaluated.
(comment (println "Test string") (* 2 3))