iOS: Apple's Safari makes it easy to view your open Safari tabs from your iPhone or iPad. That means others can see what you have open. If you want to close one of those tabs remotely, swipe to the right.
You need to register your mobile devices and computers to the same Apple ID, but after that, everything should sync. Then, pull up your tabs on the current device, pull down, and you'll see your iCloud tabs on other devices. Swipe to the right and tap Delete to close that tab on its original device.