what does 3 quotes ,""" string""" mean?

Answer 1

Those are used in Python to define Docstrings in your code. They also work to define multiple line comments.
To use them as Docstrigs, they have to be written just after the def line of your function like this:
def someFunc():
"""
Everything here is a Docstring...
It can span as many lines as needed...
Enough!!
"""
print("Hello world!)
Anywhere else, it will be just multiline comments with whatever info you want to add to your code.

Answer 2

It is used for multi line comments