The idea behind distributed computing is very simple. Back in the 70s and 80s, it was believed the only way to solve very complex problems was using super computers and hence more and more sophisticated and expensive super computers were made for research etc. etc. However, with the rise of the home PC, and as they grew more and more powerful, someone came up with the idea that, if the problems they were trying to solve were broken up into smaller pieces, all of the unused processing power of the millions of computers out there could be used for free to further research and benefit science.
This has been used in countless projects (I think SETI set up a client for PCs to analyse all the data they receive whilst they were in screensaver mode) and Folding on the PS3 is just one of these which runs on idle PS3s - the PS3 is essentially an extremely powerful computer, and the idea of using the millions of them to further the research of protein folding and the diseases involved (such as Alzhemier's and Huntigton's) around it is a good one.
Basically, if you don't want to, you don't have to do it, but by doing it, you're helping scientists number crunch the huge amounts of data to get answers to try and understand and cure these diseases. So even if you only run it occasionally, it all helps!
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