I'm studying photography. I don't understand the last sentence in this picture. What does that sentence mean?

Pinhole cameras use pinholes instead of lenses to create images. In order to ensure the imaging effect, the diameter of the pinhole is often very small, and little light passes through, so it must be matched with a long exposure time. When the outside light is relatively dark, the exposure time often takes several hours or even longer. Just like using a very thin water pipe, it takes time and patience for the water to fill a large tank.