What does "low quality" mean in photography? What about high-quality pictures?

Simply put, it is the image quality at low sensitivity and the image quality at high sensitivity.

They said:

The higher the sensitivity, the greater the exposure of the photo.

Without a flash, the weaker the ambient light, the higher the ISO required for shooting, because the shutter speed can be relatively increased to prevent the photos from blurring due to hand shaking and the dynamic subjects from blurring due to the low shutter speed. Another effect of sensitivity is that the higher the sensitivity, the more noise there is in the photo.