What's difference between Function and Method?

Both of them refer to any chunk of code that can be reused again with different arguments and may or may not cause side effects. Traditionally called a "subroutine". They are named differently because of the different context they appear within.

A method is a subroutine that is part of the instantiation of a class (an object) in an object oriented programming language, whereas a function is a subroutine not tied to any class/object. They are also referred to as free functions sometimes.
Another thing to know from the functional programming aspect is that functions have a property that they cannot cause side effects.

Method can only be called if you have an instance of some class, whereas to call a function all you need is to provide it the required arguments it needs.

Another interpretation: a method is just a function that receives the object instance as its first argument. (Ex: the self keyword in python)

A method is a function in a object oriented context.

Function is only that, method is a function inside a class.