Running multiple apps side-by-side is something that many users – especially those with tablets – have wanted on Android for a long time. And while we've seen a few implementations before (remember Cornerstone?), none have really taken off. Sure, Samsung has an option for multi-window on its more recent devices, but that's still a far-from-perfect solution, as it only allows certain apps to run together.
Given how oft-request/desired/lusted after this feature is, the devs behind the Paranoid Android ROM decided to try to bring it to life in a practical, usable way. However, it's probably not yet exactly what you're expecting, as it's little more than just pop-up windows with containerized apps held within.