Lossy compression
Discarding information the eye or ear will not notice anyway — and thereby compressing far harder than would otherwise be possible.
Lossless methods can only exploit redundancy and rarely get below a third of the original. Lossy methods use perceptual models instead: JPEG discards high-frequency detail the eye is insensitive to, MP3 discards sound masked by louder tones nearby.
The consequence is that every re-encode degrades the material, since the next pass compresses the artefacts the previous one left behind. The effect is called generation loss and is why an image saved ten times looks blocky — and the basis of the entire genre of deliberately over-compressed internet images.