Inside the tube of the turkey baster is a column, or chamber of air. When you blow over the edge of the turkey baster, its edge vibrates, compressing the air inside the tube at equal intervals. The compressed air moves down the column and bounces back toward the opening once it hits the water. By doing this, you have created a standing wave, a wave that remains at a constant position. When you change the water level, it changes the wavelength of the standing wave.
So you get to learn about sound in a standing wave with stuff you find around your house.