How do you turn off a redstone torch?

As seen in many videos, the redstone torch can be switched on:

and off:

(pic found in web)

How do I achieve the second one?

Power the block which the torch is attached to. It must be an opaque, solid, full block (so not glass, side of stairs etc; in particular glowstone and sea lanterns also count as transparent). You can power the block by running redstone signal into it (through powered redstone dust or repeater/comparator pointed into it) or by powering the block directly with a lever or a button. Similarly, attaching the torch to a redstone block will extinguish it immediately.