What exactly does the .join() method do?

Look carefully at your output:

5wlfgALGbXOahekxSs9wlfgALGbXOahekxSs5
^                 ^                 ^

I've highlighted the "5", "9", "5" of your original string. The Python join() method is a string method, and takes a list of things to join with the string. A simpler example might help explain:

>>> ",".join(["a", "b", "c"])
'a,b,c'

The "," is inserted between each element of the given list. In your case, your "list" is the string representation "595", which is treated as the list ["5", "9", "5"].

It appears that you're looking for + instead:

print array.array('c', random.sample(string.ascii_letters, 20 - len(strid)))
.tostring() + strid

join takes an iterable thing as an argument. Usually it's a list. The problem in your case is that a string is itself iterable, giving out each character in turn. Your code breaks down to this:

"wlfgALGbXOahekxSs".join("595")

which acts the same as this:

"wlfgALGbXOahekxSs".join(["5", "9", "5"])

and so produces your string:

"5wlfgALGbXOahekxSs9wlfgALGbXOahekxSs5"

Strings as iterables is one of the most confusing beginning issues with Python.

To append a string, just concatenate it with the + sign.

E.g.

>>> a = "Hello, "
>>> b = "world"
>>> str = a + b
>>> print str
Hello, world

join connects strings together with a separator. The separator is what you place right before the join. E.g.

>>> "-".join([a,b])
'Hello, -world'

Join takes a list of strings as a parameter.