What exactly is repl???

The Java REPL is a command-prompt for Java, and REPL stands for Read-Evaluate-Print Loop. The REPL is for testing Java code, but not for creating complete programs. The reason REPL isn't good for big programs is because you can't save the code and it wasn't made to do that. If you wanted to create a large program, you should probably create a Java file. But if you just want to test something basic like printing out 1 + 1 then creating a file is meaningless because you probably don't want to save that code, you just want to know the answer. So, for small tests, use the REPL. For large tests or entire programs, use Java files. You will learn more about the Java REPL and Java files later in your course, and also, the more programming you do, the more you understand about which choice to use.

Hope this helps and happy coding! ~Alex