When Apps are running you will note that they run full screen without any sign of a the old RED X in the top right of the screen. whilst minimizing the app, or opening another , puts it in a sort of hibernated state ( ie barely running but ready to start instantly) there are times you will want to close it fully.
On a touch screen....
Place your finger at the top of the screen and drag down to the bottom. When you touch the top the app shrinks and hovers at the top, as your finger reaches the base of the screen (the app will follow your finger) the app moves to the bottom and then disappears, it is now closed.
With a mouse....
Move the mouse to the top of the screen, click and hold the left mouse button, whilst holding it down drag the mouse to the base of the screen, (the app will follow the mouse) release the mouse button as it touches the bottom of the screen and the app disappears and is now closed.