A level in a video codec is the set of capacity limits that constrain what an encoder can produce and a decoder must support. It's a stricter cousin of profile: where profile says "which coding tools are used", level says "how big the numbers are". Every codec defines a ladder of levels from low (mobile-friendly) to high (8K broadcast), with explicit caps on resolution, frame rate, bitrate, buffer size and DPB size at each level. A device advertising "H.264 Level 4.1" can decode anything within those limits but nothing larger.
The canonical reference points. H.264 Level 4.1 = 1080p30 at up to ~10 Mbps — the spec that essentially every device made since 2010 supports, and the safe target for general H.264 delivery. H.264 Level 5.1 = 4K30 at up to ~62 Mbps — covers everything except extreme high-bitrate or high-framerate work. HEVC Level 5.1 = 4K up to 60fps at up to 60 Mbps — the practical 4K HDR target. AV1 Level 5.1 and VVC Level 5.1 play similar roles in their respective codecs.
For a product team, level is where compatibility errors live. The classic symptom: a video plays fine on a powerful desktop but refuses on a phone or smart TV. Cause is almost always that the encoder produced video beyond the device's supported level — too high a bitrate, too many reference frames, too high a buffer requirement. Fix: re-encode with -level explicitly set to what your audience supports (Level 4.1 for broad H.264 compatibility, Level 5.1 for mainstream 4K HEVC/AV1/VVC). Cost is trivial — usually just slightly more conservative encoder settings.
