Loads a namespace if not already loaded. It takes an argument(symbol) which should be a quoted

  (require 'clojure.string)
  ;; => nil

After requiring a namespace, we need to refer the functions in a fully qualified names. Like,

  (require 'clojure.string)
  ;; => nil

  (clojure.string/split "This is an example of require" #" " )
  ;; => ["This" "is" "an" "example" "of" "require"]

Other way is to alias a namespace at the time of requring using as.