What's a REPL process and what can I use it for?
Relation between Repl, interpreter and compiler
I've been Experimenting on heroku.com with node.js and I've heard of a "REPL process" I could use for experimenting in my app’s environment.
Could someone give me an overview of what it is, and how it might be applied?
REPL - As per the Wikipedia entry
It's a Read Eval Print Loop.
What that means for you is: It's like a console for your app. Every line you type is added to the existing program. But it's a bit more complicated than a "mere console" and it has some fancy additional things you can do.
In the case of your question, it's the node that's the REPL, and if you invoke node (like on your own machine - it's not a very large program) without any app code, you get the REPL.