Google Instant is a new type of search.
'Instant' differs from standard search in that results immediately appear while you are still typing in you search term. For example if I type the search box Music t as if I was about to search for Music Technology Google presents results immediately after I type the 't' connected with 'Music Theory' as this is the most popular search, according to Google! I'm not sure how useful this, I guess like anything new, you have to get used to how it works. Google also switched 'Instant' off automatically while at one stage "due to slow broadband speed" which I thought was a little strange. I don't have a particularly slow connection by any means.
If you would like to learn more about Google Instant and how it might affect your SEO if you have a website, Jonathan Leger has written an interesting article at http://jonathanleger.com/does-google-instant-change-the-seo-game/
Happy Searching : )