A REPL, or Read-Eval-Print Loop, is an interactive programming environment that allows users to write and execute code in real-time. It reads the input code, evaluates it, and then prints the result, creating a loop that continues until the user decides to exit. REPLs are commonly used in languages like Python, JavaScript, and Ruby for testing snippets of code quickly. REPLs are particularly useful for learning and experimenting with programming concepts, as they provide immediate feedback. They can also help developers debug code by allowing them to test functions and expressions without needing to create a full program or script.