Anything in a "temp" or "cache" folder should by definition be be safe, anything whose file extension is "tmp" should also be safe, and memory dump files and crash/error reports the same but I can't give you a comprehensive list of what's safe to delete because I don't have such a list.
I wasn't recommending you delete anything you find labelled "unknown" by windirstat as much as warning you that in normal mode windirstat can report some files as unknown, like i think it calls various windows install files, the recycle bin, the windows seach index, and that some of these files are safe to remove but can sometimes be hard.
You can run windirstat as admin and this reveals the names of a lot or all of the unknown files. Once you find they exist, if they do, then you can go looking up what they are.
But if you haven't already run windirstat or similar and found a bunch of files and folders that aren't your downloads folder, your programs folders, your windows folder then you need to do that. If you find any big files or folders you can look them up and find out what they are. As a first guess, it's the windows.old folder or downloaded windows upgrade files, like happened for me when I had to reinstall windows after a weird crash and the update files didn't delete, but of course I don't and can't know what's on your machine