When a mallet strikes a glass container, it causes the glass to shake, which shakes the air inside it and around it, which then shakes your eardrum and you hear a noise. The shape, size and composition of the glass determines how quickly or slowly the glass will shake (the thinner the glass, for instance, the faster it can vibrate). The speed of the vibrations, which we call sound waves, determines the pitch you hear. Faster sound waves create higher pitches and slower sound waves create lower pitches. Adding water makes the container heavier and harder to vibrate, so the pitch gets lower.
Crystallophone is the name for a musical instrument that uses glass to make sound. The crystallophone in this project is a percussion instrument, like a xylophone. Other types of crystallophones include glass harps and glass harmonicas.