Unless they just changed it, they seem to have snappy taglines that explain it well already:
Liveloop is really real-time collaboration within Powerpoint
Collaborate in real-time, keystroke by keystroke, without ever leaving Office.
These aren't bad. I wonder if they have tested them with the target market.
I have sympathy. It's a hard concept to get across.
That said, it takes until 2:30 in the video before we see people collaborating on the same slide at the same time. And there might be a bit of meta-confusion because they're working on a presentation for "LiveLoop Zoo". I know that seems like a harmless self-reference, but I've seen people get tripped up on much easier ideas.
That seems a bit wrong. I wonder if, instead of a video where I have to press "play", we could have a silent animation that starts immediately on the home page. Show two cartoon figures typing into PowerPoint screens, with their changes reflected immediately. (And it's never bad to show human faces with emotions.)
Anyway, that sort of thing is hard, and they're to be commended for getting this far, my armchair criticism aside.
How about doing hallway usability testing? Show the page to people in the target market for five seconds and then ask them "what does this company do?" Five of these will tell you everything you need to know.
If you don't have a constant stream of fresh users walking through your offices, you can do something on http://www.usertesting.com/ .
A/B testing understanding might not be feasible, but in the end, they are trying to sell their plugin so A/B testing for signups or sales or something like sounds like a good correlation to understanding.
About 20 seconds in, your video says:
Liveloop is a simple powerpoint plugin that lets everyone work on the document at the same time, seeing everyone else's changes as they type.
That is the quickest way to understand what you do. If I want to see it in action, I can click through to a video demonstration.