The Arecibo telescope in Arizona watches the sky for extra-terrestrial broadcasts. It’s the largest telescope in the world, and has recently been upgraded to generate more than 500 times the previous amount of daily data. SETI now needs to sift through a daily 300gb, and they therefore need more users to install their software, which analyses batches of data whenever your computer isn’t doing anything else. This is a million miles from UFOs and all that nonsense - it’s actually incredibly likely that civilisations exist elsewhere in the universe, it’s just detecting them that’s difficult - so if your computer is wasting time doing nothing, why not head on over and give them a hand?