The lower you bring down that number, the lower the quality of the picture. Here’s an illustration…
Here’s one made with an actual photograph, taken from this thread.
You need to view it at 100% to really see differences correctly, here’s a link to view it fullscreen…
Obviously the “Quality 0” is an extreme example, but it shows the kind of processing that’s going on. Take a look at the background color as the quality decreases, it changes from an off-white to light pink, to light green, to grey, to white.
Also of interest are the file sizes…
