The JPEG "quality" slider controls how aggressively detail is discarded. 100% keeps the most detail (largest file); lower numbers throw away more (smaller file).
The sweet spot
For web photos, 75–85% is the magic range: the file shrinks dramatically while the loss stays invisible to the eye. Below ~60% you start seeing blocky artifacts.
Why higher isn't always better
Jumping from 85% to 100% can double the file size for a difference almost no one can see — wasted bytes that slow your page.
The best workflow: pick ~80% and watch the live size. The compress tool shows the output size as you drag, and our compress-to-size guides hit exact targets. Related: lossy vs lossless.