The term REPL stands for Read-Eval-Print Loop. Even the C language has REPL implementations these days, but the first REPL was a Lisp invention. Jwno inherited the tradition of providing this great toy tool.
Jwno's REPL have a coding environment that's almost identical to the one you get in the config file, so you can run your experiments in the REPL, then incorporate them into your config file, and your code would have pretty much the same behavior.