What is it?
-----------
(a little bit more in detail)

You are running a lot of applications for Your customers. (I use the word 
customers because as a system administrator You allways have customers,
no matter if they are employees or customers of the company You are working
for.)
Each application needs a few or a lot of components (like webservers,
application servers, DNS- or mail servers, LAN- or WAN-connections ...)
to work properly.
There are components You need for only one application, and of course there
are components which are important for more applications (e. g. DNS-servers)
You already are running Nagios to monitor all of these components I guess.
(Otherwise You would not think about a Nagios AddOn.)

If You are the only system administrator of Your company, You will probably
know all Your applications very well, You know which application needs 
which components - then You will not need this AddOn.
If there are more admins, You probably will share work. This means each admin
knows view applications very well and the other applications only a little 
bit.
So maybe You would find it great to visualize, how all these components work
together. If one ore more components fail, You want to see on one single 
page, which applications are unavailable for Your customers - in this case:
Install this addon.

It has two modes:
1. Nagios Business Process View
   it shows the actual state of Your applications
2. Nagios Business Impact Analysis
   this is a simulation mode.
   You can set each of Your components to every stat You like.
   So if You want to know: What would be if my web server would fail now?
   Just klick the state of Your web server an set it to CRITICAL
   Return to the overview page and look, which applications are now in
   state CRITICAL. 

The states of the single services and hosts defined in Nagios are taken from
the NDO database.