[Tutor] What is the square bracket about?

[Ivan Low]

def adder1(*args):
     print 'adder1',
     if type(args[0]) == type(0):
         sum = 0

     else:
         sum = args[0][:0]
     for arg in args:
         sum = sum + arg

     return sum

Hi, I was trying to understand what this function is all about.
I didn't tried the adder1 function which result in adding number or  
text together which I think there is an easier way to do it right?  
Why go through all the way to write a function like this?

The square is a big mystery to me. I can't explain why it is there.
I tried sum(1,2,3) and sum('a','b','c') it wouldn't work. I even  
search the document and I can't find anything there.

Ivan