What is a video codec, in simple terms?
A codec is the compression recipe that shrinks video small enough to store and stream — raw video is absurdly large (gigabytes per minute), so a codec like H.264 keeps only what your eye needs. The container (MP4, MOV) is just the box the compressed video and audio travel in; that's why 'an MP4' can behave differently depending on the codec inside it.
What you practically need: H.264 in an MP4 plays everywhere and is what platforms recommend — when in doubt, export that. H.265/HEVC compresses ~40% smaller but has patchier support. Editors like EseCut handle this at export so you never hand-pick codec settings.
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