Watching my beer ferment is a fascinating thing to me. A by-product of yeast fermenting is CO2, which is what you see pushing the bubbles through the tube. This tube is called a “blow-off” and is an outlet for escaping gases. It prevents those gases from creating too much pressure in the fermentation vessel (this one is glass; too much pressure inside glass is a very bad thing).
The beer fermenting here is a Sierra Nevada Pale Ale clone that I’m looking forward to trying.