REPL is an abbreviation for "Read Eval Print Loop". First it reads your expression, then it evaluates your expressions and prints the results in the environment that you are using.
Here's a simple REPL implementation:
(define (repl) (display "> ") (display (eval (read))) ;; read, eval, print (newline) (repl)) ;; loop :)
You can hose-the-repl rather easily.