Lossy compression permanently removes data deemed unlikely to be noticed by human perception. The removed data cannot be recovered — once you've saved a JPEG, the discarded detail is gone. However, at high quality settings (80-90%), the removed data is genuinely imperceptible, and the file size reduction is dramatic: a typical 5MB PNG photo becomes a 500KB JPEG at 80% quality.
Lossy compression exploits perceptual models: humans are more sensitive to brightness than color (so JPEG compresses color more aggressively), more sensitive to low-frequency patterns than high-frequency detail (so JPEG discards fine detail first), and less sensitive to distortions in complex textures than in smooth gradients. These psychovisual models guide which data is safe to remove.
An important rule: never re-compress a lossy file. Each compression cycle discards more data, compounding quality loss. If you save a JPEG at 80%, then save it again at 80%, you've effectively applied compression twice. Always start from the highest-quality source available. FileCurve's compressor shows the output file size and quality preview before you download, helping you find the right balance.