The discrete cosine transform (DCT) is the per-block frequency transform at the heart of nearly every mainstream codec - H.264/AVC, HEVC, AV1. After a frame is split into blocks, the DCT rewrites each block's pixels as a sum of wave patterns, from a flat average-brightness wave up to fine ripples; smooth blocks need only the first few waves, detailed blocks need many. Representing the block as waves is what makes the next step possible: quantization can round off or delete the small high-frequency coefficients to save bits. Crucially, the DCT is computed on each block independently and never models correlation across block boundaries, which is the root of blocking - two neighbors can round to two different flat tones with a visible seam between them. The transform is also why ringing appears, since a sharp edge packs its energy into the highest-frequency coefficients that quantization truncates first. The DCT itself is lossless and reversible; the loss happens in the quantization that follows it.
