Sound is simply vibrating air. These vibrations propagate as waves. The higher the frequency of that vibration, the higher the pitch of the sound, but the shorter the wavelengths.

The vibrations here are caused by the splashing of the water. These vibrations have tons of different pitches, but when you have air vibrating in an enclosed space the sound waves reverberate within that space which amplifies certain pitches, namely the ones with wavelengths that match the size of the space. As the space for the air to resonate gets smaller, the shorter the wavelengths are that get amplified.

This is exactly how brass instruments work. When you move the slide of a trombone out the tube gets longer so you get a longer wavelength and thus a lower note.

That's the basic explanation, but here is a more advanced explanation of why the wavelength is dependant on the space it can vibrate in, even when it has an open end as a glass has. Many differently shaped rooms have different effects on timbre and pitch. When we talk about this we tend to simplify it by talking about a tube that is open in one end, which conveniently is exactly what most glasses are.

The bottom of the glass is where the air vibrates the least freely, so here it forms what's called a node. You can imagine it as the ends of a jumping rope that barely move, while the center where there is most movement, is what's called the antinode.

If the tube of air were sealed in both ends there would be a node in both ends and an antinode (or several) in the middle just like the jumping rope, but when the tube is open in one and there will always be an antinode in that end where the air can vibrate the most freely.

The fundamental pitch of a tube is the biggest wavelength that can "fit" in it. For a tube that's sealed in both ends that is simply a wavelength the exact length of the tube, but for a tube that's open in one end the fundamental wavelength is twice the length. That's because you have an antinode in one end so it's almost as if only "half the sound wave" is inside the tube.

For a closed tube, the overtones of the fundamental simply have 1/2, 1/3, 1/4, 1/5 etc. the wavelength, because you'll always have a node in either end, but you can have more nodes and antinodes throughout the tube. But for a tube with an open end they are 1/1.5, 1/2.5, 1/3.5 etc. because you'll always have an antinode in one end.