A REPL or read eval print loop is what we called an interactive prompt back in the day when I learnt Python and Ruby.
It is a awesome tool when it is combined with the right language. For example, I love it with node or Clojure; but I hated it with Python. And it doesn’t make that much sense with C# or Java.
For a REPL to make sense you need to be able to test small chunks of code. Like this function or this expression; or my typical thing, “would this work” or how the fuck was that syntax again?
In other words, if your language require a lot of “foreplay” to run code, like declaring a namespace and a class etc (I’m looking at you Java and C#) it will probably not be the right thing. But if you can evaluate code without much fuss it is. Another thing that is really important is that you need to be able to just paste code into there. This is something that always drove me nuts about the Python REPL. It is so interactive that everything gets fucked up when you paste crap.
REPL’s are awesome, and can make your development really fast, if combined with the right language.