It's a circuit breaker that's "popped." They sell for about $30.
It apparently contributed to this, which happened a month ago today (and which I wrote about here):
The Polish airliner left Newark and noticed right away there was a leak in the plane's hydraulic fluid. That's when the pilots made mistake #1, according to a report that was just released (available here). They decided to continue flying to Warsaw, where they found out the plane's landing gear wouldn't extend.
An alternative landing gear extension system didn't work because of that $30 part shown above.
Had the pilots noticed the circuit breaker, they could've pushed it in, lowered the gear, and landed the plane.
But these days, airline crews depend on computers to tell them what's wrong and the Boeing airplane wasn't built to tell pilots when a $30 circuit breaker had popped (pilots of smaller airplanes have an old-fashioned fail-proof system: They touch run their fingers across each circuit breaker to be sure they're all engaged before taking off).
When authorities lifted the plane off the runway, pushed in the circuit breaker, applied power, and flipped the landing gear switch, down came the landing gear.
And that's the simple sort of thing that causes major air disasters.