What is the source code of the "this" module doing?

It uses ROT13 encoding. This is used because it's a joke.

You can also use Python functions to decode string.

Python 2 only:

import this
print(this.s.decode('rot13'))

Python 2 & 3:

import codecs
print(codecs.decode(this.s, 'rot-13'))

This is called rot13 encoding:

d = {}
for c in (65, 97):
    for i in range(26):
        d[chr(i+c)] = chr((i+13) % 26 + c)

Builds the translation table, for both uppercase (this is what 65 is for) and lowercase (this is what 97 is for) chars.

print "".join([d.get(c, c) for c in s])

Prints the translated string.

If you want to make the ROT13 substitution by hand - or in your head - you can check that because 13*2 = 26 (the number of the letters of the English alphabet), it's essentially an interchange:

a <-> n
b <-> o
c <-> p
...
m <-> z

A <-> N
B <-> O
C <-> P
...
M <-> Z 

Vs lbh cenpgvfr ybat rabhtu, lbh'yy riraghnyyl znfgre gur Mra bs EBG-13 nytbevguz naq ernq guvf Xyvatba ybbxvat grkgf jvgubhg pbzchgre uryc.