What it holds
The bytes received so far, in the order the browser saved them. When the download completes, the browser removes the extra extension and the file gets its real name.
A leftover movie.mp4.crdownload is therefore usually the beginning of movie.mp4, with the rest missing.
What to try first
Look on the browser's downloads page (chrome://downloads). If the entry offers Resume, use it: the server may be able to send the missing part. Resuming depends on the server, and many links expire.
When the source is gone
Renaming sometimes opens the file, but most formats keep information a cut file lacks. An MP4 often stores its index at the end, and a ZIP keeps its directory there.
That is the case Crdownload was built for: it reads what arrived and writes a valid file with those parts.
Common questions
Can I delete a .crdownload file?
If the download is finished or no longer needed, yes. While the download is active, the browser is still using it.
Is it dangerous?
It carries the same risk as the complete file would. Treat it according to where it came from.
Sources
Documents consulted for this guide.