iCloud Tabs allow you to access web pages you have open in Safari across any iPhone, iPad, or Mac you've got synced up with your iCloud account. If you're at home browsing on your Mac and later need to access a page you had open on that Mac from your iPhone or iPad, there's no need to scour the web for it, just use iCloud Tabs to instantly view the page on any of your iCloud enabled devices.
iCloud tabs doesn't have its own setting to enable but if you don't have iCloud syncing with Safari turned on across all your iOS or OS X devices, you won't be able to use iCloud Tabs.
iCloud Tabs are only available in OS X Mountain Lion, so if you are on an older version of OS X, unfortunately, you won't have the ability to use iCloud Tabs on your Mac. If you are on Mountain Lion, just perform the following steps to make sure iCloud Tabs are enabled.