From the github page: “Shim is a node.js-based browser-compatibility tool that lets you synchronize several devices/browsers and surf the same pages simultaneously on all of them. ”

Or, in other words, you can browse a test site on a local network from different devices: iPhone, iPad, any laptop, Samsung Galaxy Tab, Kindle, etc. and they all see the same things at the same time. If you move to another page from one of these devices, all the device connected move to the same page, allowing you to test all of them at once.