When you download a video directly from Reddit's servers or use certain third-party downloaders, it retains this alphanumeric "source" string as the filename. How to identify the content
If you recently downloaded it, check your Reddit "History" or "Saved" tabs in your profile.
Since the filename is an internal server ID, there isn't a public "directory" to look it up. However, you can try these steps to find the original post: