Redstone Reapeater

Redstone Repeaters can extend the current of a redstone wire beyond the 15 block limit, or to delay the incoming signal.
The repeater also acts as a delay. The switch can be toggled by clicking it and has 4 settings: 0.1 second delay, 0.2 second delay, 0.3 second delay and 0.4 second delay.
Input can come from a powered block, another repeater, a switch, a redstone torch or redstone wire. Note that the redstone wire must be wired directly into the correct side of the repeater. Wire laid across the input side will serve as input.

Now you know what they do you can use them to extend power when you need Redstone to travel a long way or use them in a 4 by 4 grid with Redstone torches to make a Redstone clock.