When you save an image you choose between two kinds of compression. Knowing the difference keeps files small without wrecking quality.
Lossless
No data discarded; pixel-perfect. PNG. Larger for photos.
Lossy
Permanently discards data for much smaller files. JPG and WEBP.
Which to use
Photos → lossy ~0.85; graphics/transparency → lossless. That is why the quality slider is disabled for PNG in the converter.