Short answer
This tool repairs corrupted MP4 and MOV video — camera and screen recordings that won't open or won't play because their structure (the index, the moov atom, a truncated or unfinalized container) is broken. It runs entirely in your browser: the file never leaves your device and 0 bytes are uploaded, which you can confirm yourself in the Network tab. The honest limit: it rebuilds structure around footage that's still in the file, but it can't regenerate frames that were never written to disk or were later overwritten.
How video corruption actually works
A video file is two things: an index that tells the player where every frame lives, and the stream data — the compressed frames themselves. Corruption almost always hits one or the other, and the difference decides everything. Index damage (a recording that was never finalized, a truncated transfer) leaves the footage physically intact in the file; the index can be rebuilt by scanning that data, and the video comes back. Stream damage (overwritten bytes, bad sectors) is different: structure can be rebuilt around the gap, but the missing frames are gone — no tool can regenerate footage that isn't in the file. That one distinction is why some "hopeless" files repair perfectly and some pristine-looking files don't.
Pick your exact problem
One generic wall of text can't fix eight different failures. Each page below covers one, with the verbatim error text, the mechanism behind it, and an honest read on repairability.
By format
By error
Why no-upload matters for video
Video is where the upload model breaks down. Files are huge (a five-minute 4K clip can pass 2 GB), so upload-first tools make you wait through a long transfer before they can even tell you whether the file is fixable, and multi-gigabyte uploads fail on ordinary connections. Footage is also often private: client work, family moments, material under NDA. In-browser repair sidesteps both problems: diagnosis starts in seconds because the file never travels, and privacy isn't a policy, it's a mechanism you can verify yourself in the Network tab.
What this can and can't fix
The general rule: repair rebuilds structure around surviving data. If the frames are in the file, a broken index, truncated container, or damaged header can be worked around, and the result is your real footage. If the frames were never written to disk (the seconds still in a camera's RAM when the battery died) or were later overwritten, they cannot be regenerated by any software, ours included. Files that read as 0 bytes are a recovery problem (getting bytes back from the storage device), not a repair problem; do recovery first, then repair what it returns. And if a repair fails, you're never charged for it.