These file sizes only apply to this image of a bookshelf, the image size of 640 x 480 pixels, and the quality % settings for the Graphic Converter image editor on the Macintosh. However, the shape of the curve shows how file size generally increases as the quality percent increases (or, as the amount of compression decreases).
At the low quality end, small increases in quality give small increases in file size, and don't appear to make any visible difference. At the high quality end, small increases in quality give very large increases in file size, and don't appear to make any visible difference. Around 50% is the "sweet spot" to get a good quality image with a small file size.
For images on the web, 640 x 480 pixels at 72 pixels per inch is small enough to show the whole image without scrolling on most monitors, and big enough to show most of the image detail. At that image size, 50% quality gives good image quality at a file size of around 50KB.