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

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.

It's a substitution cipher, rot13.

It's a substitution cipher (as mentioned in previous answers). Historically speaking, it's the Caesar cipher.

https://www.google.de/search?q=caesar+cipher&cad=h