"Nasal", that's FlightGear's built-in scripting language - it looks and works pretty much like JavaScript, or any other C-like language (C, C++, Java). So if you know any of these languages already, it should be fairly easy to get started.
People already familiar with other scripting languages such as Perl, Python or Ruby should also find it easy to learn Nasal quickly.
The Nasal documentation is maintained in the FlightGear wiki: http://wiki.flightgear.org/index.php/Nasal