Argument in a function

Hey guys, I am new with Python and my question is (based on the following code snippet representing a program displaying any vowels found in a supplied word): how does Python know what 'word' argument is (how does Python know what 'word' argument means) since 'word' was not defined anywhere in the code:

def search4vowels(word):

     vowels = set('aeiou')

     found = vowels.intersection(set(word))

     for vowel in found:

         print(vowel)

thanks