What is this Indentation Error?

I'm getting this message as a result and I don't know what it means. My code so far is:

#Your optional code here
#You can import some modules or create additional functions


def checkio(number):
    #Your code here
    #It's main function. Don't remove this function
    #It's using for auto-testing and must return a result for check.
if ((number%5==0) and (number%3==0)):
    return "Fizz Buzz"
else if (number%5==0):
    return "Buzz"
else if (number&3==0):
    return "Fizz"
else return str(number)
    #replace this for solution


#Some hints:
#Convert a number in the string with str(n)

#These "asserts" using only for self-checking and not necessary for auto-testing
if __name__ == '__main__':
    assert checkio(15) == "Fizz Buzz", "15 is divisible by 3 and 5"
    assert checkio(6) == "Fizz", "6 is divisible by 3"
    assert checkio(5) == "Buzz", "5 is divisible by 5"
    assert checkio(7) == "7", "7 is not divisible by 3 or 5"

And then I get the following error messages: IndentationError: expected an indented block (, line 9), ?, ? Fail: checkio(undefined)

What does each one of these mean? When I try to run in a normal compiler it works properly!