The filename appears to be a specific identifier typically used by Reddit's video hosting service (v.redd.it) or similar social media content delivery networks .
If you have the file and want to know what it is without opening it: 28821-_1531966025.mp4
If you have the link it came from, search for the folder ID (the 28821 part) on Google to find the original post. The filename appears to be a specific identifier