What does “return” do? Does it repeat a value?

Question

I replaced 0 with “Main()”, and it popped out “Hello World!” many times!

Answer 1

return - returns the value of a function so that you can do something with it, i.e. assign it to a variable

Answer 2

The phenomenon which occured in your code is called recursion
https://www.sololearn.com/learn/CPlusPlus/1641/