I used to do SETI@home, but haven't for years now.(no particular reason...)
let me try and translate for other folks, based on my experience With SETI@home:
S.E.T.I., has tons and tons of raw data coming in that needs processing. you and others like you install their software on your pc. It then downloads a packet of data from the server, and processes it during your pc's Idle periods. then when it's all processed, uploads it, and gets another.
last i looked they even had programmed it to run on your GPU, rather than CPU.(can do more simultaneous calculations=handles more data faster)
you are basically volunteering some of your computers resources to scientific advancement. and helping them keep equipment costs down.