UnblockUs is basically a bunch of proxies located in other countries that help you bypass some websites regional licensing issues.
Basically what this means is some sites like Netflix™ can only show you particular shows in the US because of licensing agreements with the copyright holders of the show. So while people in Mexico may be able to enjoy streaming episodes of The Fresh Prince, we in the US, are unfortunately unable to view them.
This software uses servers set-up in those countries to make your visits to services like Netflix appear to be coming from other countries, thus releasing you from the burden of obeying regional copyright issues.
Is this legal? YEEEEEEEnope, probably not -- but I'm not sure if there has been any legal precedence for using proxies to stream content like this. Use at your own risk.