The DPB — Decoded Picture Buffer — is the memory area inside a decoder that holds recently-decoded frames so they can be referenced by future frames. When a P-frame says "my content is mostly the same as the frame from 200 ms ago, shifted slightly", the decoder needs that older frame still in memory to reconstruct the current one. The DPB is where those reference frames live. Modern codecs allow up to 16 or so frames in the DPB simultaneously, with the encoder freely choosing which ones to keep and discard.
Why this matters at all: there's a hard limit on how big the DPB can get, set by the codec profile and level. H.264 Level 4.1 (a common 1080p setting) allows a DPB of 4 reference frames at 1080p resolution; HEVC Level 5.1 allows more; AV1 has its own constraints. Exceeding the DPB limit means the decoder cannot keep up; the encoder must respect this when planning its reference structure. This is a meaningful constraint because more reference frames generally mean better compression — but the codec's profile and level cap how many you can use.
For a product team, the DPB is invisible plumbing that shows up in compatibility errors. The typical symptom: a video encoded with a higher level than the target device supports — say HEVC Level 6.0 — refuses to play on a phone or smart TV stuck at Level 5.1. The encoder used more reference frames than the device's DPB can hold. Fix: re-encode with the level constrained to what your audience actually supports (most encoders accept a -level parameter). For mainstream 4K streaming, HEVC Level 5.1 / AV1 Level 5.1 / VVC Level 5.1 cover essentially all modern devices.
