The floating icons appear above the active application. It means you can carry on chatting without having to bounce between applications.
I need to wait and see how intrusive they are, but based on the preliminary glances shown today at the event, I think they are cool.
However, in order to have Chat Heads, you have to download all of Facebook Home, and I definitely don’t want to sacrifice my whole home screen to Facebook for the feature. I want Chat Heads to be offered unbundled, but this — obviously — does not correlate with the interests of Facebook.