Receivers can emit power to neighbouring blocks.
Certain blocks will affect the state of neighbouring Transmitters (or any block that reacts to power, like doors, lights etc) via power emission. Initially these will be Pressure Plates and Switches.
e.g. When a switch is hit, it will toggle the state of the neighbouring Transmitters. The Transmitters will send a signal with their new state. The signal will be received by all Receivers in range on the same frequency. If the signal is off (zero), the Receivers will stop emitting power. If the signal is on, the Receivers will emit power. Any neighbour blocks of the Receiver that can react to power will update accordingly, like a Door will open (if powered), or close (if unpowered). A powered light will turn on if powered, or turn off it not, etc. 
- It's the first item/block to use radio waves to cause a signal instead of the coventional uses of wiring or piping. Read more at: Transmitter Wikipedia Page