The name REPL stands for Read-Eval-Print-Loop - an interactive, typically console-based, programming environment. However, repls at Repl.it have exceeded that definition and are now capable of much more.
A repl at Repl.it is an interactive programming environment. You can create a workspace in any number of languages, where you are given a container on a virtual machine where your code can run. In any given repl, there are two main parts - the editor and the console.
The console is where the standard output will appear, and where you will be prompted to enter standard input. In most languages, the console doubles as an interactive prompt - you can evaluate snippets of code interactively, and in some cases, you can interact with variables that have been defined in your main body of code.