Lossy vs lossless
JPG and WebP use lossy compression: they discard image data the human eye is unlikely to notice in exchange for dramatically smaller files. PNG is lossless — it never throws away data, which is why it stays larger. The quality slider controls how aggressively lossy formats discard data.
Reading the slider
Higher quality means a larger file that's closer to the original. Lower quality means a smaller file with more visible artifacts. For most photos, 70–80% removes most of the file size while staying visually identical. Below about 50% you'll start to see blocky artifacts in skies and gradients.
Target file size
Some tools let you aim for a target file size instead of a quality percentage. IMGifyx will iterate locally to get as close as it can to your target while keeping quality as high as possible — useful when a platform enforces an upload limit.