"What's and airlock, and why would you sniff it?"
An airlock is placed on a fermenter to prevent nasties(bacteria, mould) from attacking the wort (unfermented beer). The air lock allows the CO2 produced by fermentation to escape, while not allowing any air to enter.
The one pictured in "Air Lock Sniffer" is a three-piece air lock. You fill the main compartment with water. The centre piece will float up while being filled with CO2, until its tipping point is reached. It then tips slightly to one side, releasing the CO2, and starting the process over.
Why would you sniff an air lock? Because at it's best, it's the scent of heaven. Fresh hops mixed with yeasty goodness.