在Javascript中在服务器和客户端之间共享代码意味着什么？

I will be honest in saying that I did not quite understand one concept while digging into NodeJS. And this concept is about how NodeJS lets us share code between the server and client. What does this really mean? Does it mean that I can write a function that I perhaps call on the client side and it gets invoked as a remote method or does it mean that the code gets copied onto the client side and gets executed?I will be honest in saying that I did not quite