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))