When beer ferments, it forms a layer of foam on the surface called krausen. Depending on what style you are brewing, this krausen can grow to be quite tall and can get into your air lock and clog it up. If this happens, the CO2 produced by the fermenting beer has nowhere to escape. It will eventually build up enough pressure to violently pop the stopper and airlock out of your carboy, and create a homebrew volcano in the process. This will cause lost beer, a mess, and an angry spouse. To avoid this you can use a blow off tube.

A blow off tube is a thick hose that goes from the top of your carboy, into a jar of water or sanitizer, creating a big air lock. This lets CO2 escape, but the hose is less likely to become clogged than an air lock. When the krausen dies down, you can switch back to an air lock. Most blow off tubes are 3/8" hoses stuck into a drilled stopper, or 1/2" hoses over a carboy cap.

This is what I used to use with carboys:

You'd have to have some very serious krausen to clog this guy! It's three pieces of 1"ID tubing slipped over two 1/2" PVC elbows, and shoved directly into the opening of the carboy. The jar with water is not shown.

I liked it because it stays very close to the side of the carboy, and you didn't have to wrestle with a big loop of hose. It was also very easy to clean.