Answer 1

When you do something like:

str(9), what you're doing is to cast the numeric type to a string. So 9 it's converted to "9".

Example:
print(str(9))

You can even check the types:
num = 9
str = str(num)

print(type(num))
print(type(str))

Answer 2

It's a function that returns the version of a string of what you pass to it.