The basic idea is, you can access files on your computer quickly using an abbreviated search that learns as you use it. So, you type 'IE' and hit Return and Internet Explorer runs - and the next time you type 'IE' AppRocket remembers how often it is accessed and ranks it higher or lower based upon that.
So, you have a hotkey to access AppRocket, type a few keys to find a document, MP3 file or an application and hit return. After a while it simply becomes a twitch on your keyboard to access any file on your computer.
I hope that makes sense, it's actually quite hard to explain in a sentence or two. It is really useful though - though I am one of the authors so I might be considered biased

If you would like some more explaination, please let me know. We are a relatively new company trying to make really nice software, we haven't gotten down the marketing bit yet...