Tablet mode lets Windows move seamlessly from a traditional windowed environment to a full screen environment to make easier to navigate Windows 10 using touch.
When using the Tablet mode, the desktop fades away and everything goes into full screen, apps will launch and stay completely expanded. You will see a new Start menu that covers the entire screen much like the Start screen in Windows 8.1.
There are two ways Tablet mode can be enabled:
Enable Windows 10 Tablet Mode via Action Center
The easiest way to enable Tablet mode is by accessing the new Action Center (Windows key + A) and then click on Tablet mode from the quick action tiles.
Follow these steps to enable Windows 10 Tablet mode via Settings
There are some devices like the Microsoft Surface which have the ability to switch to tablet mode automatically when a keyboard is not attached.