So JavaScript was made to run in a browser. You click a button or whatnot and some code runs. But people liked it so much that that they made an external runtime outside of the browser, so you can run code just like any other language, like running a Python script. Nodejs also has a bunch of server specific functions such as file reading and writing that JavaScript running in the browser would not have access to. If you’re asking what can be done with it, you can use it to run a JavaScript script one time, or you can use it to run a web server.