Topic of recursion lect-2

#1

I am unable to understand this important line i.e"Functions works before writing function definition"…
Can you please explain this in easy way ?

#2

@Abbas786 Bro that means say you have a function called sum, now its not written by you but you pass two numbers to it, you believe that you will get the sum right?
No matter whatever be the input is!

So recursion mein ye maan ke chalo ki jo hamara recursive function hai wo har input ke liye chalta hai no matter what!