Beta-testing is kind of like what Boeing is doing right now with the Dreamliner, putting it into the hands of a few people who can fly it under real-life circumstances, seeing if there are any bugs to work out, or if anything breaks.
In the case of software, like a flight tracker, a set number of people representing several different operating systems, browsers, etc. would be tasked with putting the software through all its functions, making sure everything works correctly before unleashing it on the general public. It's much better to find & fix issues with a small control group, rather than hearing about problems from 6000 people who may or may not document well enough to easily fix the situation.
Edited to add: This may be the only time I'm actually able to ANSWER a question here.