Remuxing takes the already-encoded video and audio streams out of one container and writes them into another without touching the encoded data. MOV to MP4, MKV to MP4, stripping an unwanted audio track — all remuxes.
Its two properties are the whole point:
- It is lossless. The bytes of the video stream are copied. There is no re-encoding, so there is no generation loss.
- It is nearly instantaneous. The work is proportional to file size, not to video length or complexity. A 4 GB file remuxes in seconds.
When it is the answer:
- A compatibility problem, not a size problem. A player refusing an MKV whose contents it could decode perfectly well. Remux to MP4 and it plays.
- Removing tracks. Dropping a commentary track, a subtitle stream, or audio entirely.
- Fixing metadata, rotation flags and faulty headers.
When it is not:
- Anything about file size. Container overhead is a percent or two. If you need a smaller file, you need to re-encode.
- Anything the target container cannot hold. Remuxing an exotic codec into MP4 may produce a file that is technically valid and still unplayable, because the problem was never the container.
The common confusion is worth stating plainly: "convert MOV to MP4" is frequently a remux, completes in seconds, and correctly changes nothing about the size. That is not a tool failing. That is the operation working.