Smartthings allows you to create a virtual device which has no physical existence. It’s just an entry in your smartthings account. But once you’ve created it, the system will treated exactly the same as a physical device. So you can make a virtual switch which can be turned on and off by other events in the SmartThings system, or even by external systems like IFTTT or harmony.
Once you have created a virtual switch, it will show up in any list of devices for your smartthings account, including in smartapps like core.
And you can use Smart Lighting to have a physical switch get turned on and off when the virtual switch gets turned on and off. This is what we mean by “follows the virtual switch.”
And in case you want to do more than just turn one other switch on and off, there are also two smartapps available in the marketplace section of the smartthings mobile app that are very useful with virtual switches. These have been around for a long time (so long that they use the older terminology of “home phrase” rather than routine, although it’s the same thing) and are very popular. One lets you tie a switch to a mode change, and the other lets you use a switch to run a routine. And, as mentioned above, that switch could be a virtual switch. 
So it’s very common to use virtual SmartThings switches with IFTTT because it allows you to have one IFTTT recipe which actually causes multiple smartthings events because you have the recipe turn on a virtual switch and then you have various things tied to that virtual switch on the smartthings side.