Why do we say an after NOU? For example, se nou AN retounen nan kous la? (se as in sister...sorry I don't know how to put the accent) Mesi

It's the Haitian Creole possessive adjective.  Sometimes we add a definite article after it.  The definite article may be a, an, la, lan or nan depending on the ending of the word that comes before the article.
Remember that these possessive adjectives may be written out completely or they may be contracted as you will see in the next few examples:

1.
sè nou an → our  sister (with a definite article)
or
Sè n nan → our sister (contracted, with a definite article)
or
sè nou → our sister (without the definite article)
or
Li se sè n. → She's our sister (contracted, without the definite article)
(all the above translate into one thing → OUR sister)

2.
pitit nou an → our child
or
pitit nou → our child
or
Pitit n an (You cannot have a contraction after a vowel)
see link about contractions

3.
Pitit gason nou an malad.
or
Pitit gason n nan malad.
or
Pitit gason nou malad
or
Pitit gason n malad.
Our son is ill.

4.
Peyi nou an gen anpil montay.
or
Peyi n nan  gen anpil montay.
or
Peyi nou  gen anpil montay.
or
Peyi n gen anpil montay.
Our country has a lot of mountains

And this doesn't happen with "NOU" only.  It happens to all the other possessive adjectives... mwen, ou, li, yo, ...
Here are a couple more examples:

5.
chapo mwen an
or
chapo m nan
or
chapo mwen
or
chapo m
my hat

6.
manje ou a
or
manje w la
or
manje ou
or
manje w
your food

7.
sè li a
or
sè l la
or
sè li
her/his sister

8.
liv yo a
or
liv yo
their book


Haitian Creole ↔ English Reference, Look up Haitian Creole and English Words