REPL stands for Read, Eval, Print, Loop. It is also known as the Python Interpreter or Interactive Shell, and it refers to a Python environment that processes your code as soon as you press enter. First, it Reads your code, then Evaluates what you wrote, then Prints the result (if there is a result), and finally Loops back to the initial prompt, allowing the user to start the loop again.
The REPL is useful for testing code on the fly, seeing if it works or what will happen. Since the REPL doesn't save your data after each session, though, you need a text editor to write and save longer scripts. Think of the REPL as a sandbox or playpen for exploring Python commands and expressions in the moment.
To use the REPL, just type python into your interpreter. You'll know you're there when you see the three >>>. To get out of the REPL, press control-d or type quit().
See this beginner's tutorial on the REPL from Learn Python.
Check out this intermediate article on the REPL and what it can do on Code With Mu.