UA: IT is basically a description of your web browser. In this case it is MicroB N900 but when you connect from your desktop it could be Internet Explorer Firefox safari etc. (will also identify Windows Linux OSX etc)

All web browsers by default tell the website they are visiting what user agent you are using. Some websites use that data to decide to send mobile or desktop versions of their site.

Very few sites actually search for the N900 user agent. But clearly you have found one.

The software you have been told to install adds an option to change the User Agent details. IE The web browser will lie to the web page and not tell it it is a N900.

The website will send the default desktop page in response.

You will have an option in settings to turn this on and off.

To install it follow the above advice on the extras testing repository.

I hope that all makes it clear.