Tessel listens for a clap, then toggles a relay to turn something on and off.
(Also view this project/get the code easily at the Tessel projects page)
A relay acts as a switch– either connecting ('on') or not connecting ('off') the two wires fed into the channel.
Use a pen or a nail to press down on the top of the relay, and feed in the red (power) wire.
The ground wire doesn't go into the relay; it just goes straight to the power adapter.