Nautilus is actually inspired by Apple's Mac OS's file manager without a doubt and if you want even more simpler "versions" of Nautilus then other file managers such as Thunar and Elementary are quite good too. Even the Unity desktop still uses the Nautilus as the primary file manager in its desktop as well.
Unlike with MS Windows where it save the thumbnails of files it generates within the each folder itself, Nautilus on the other hand stores all the generated thumbnails of videos, pictures, etc in a single folder. Now in comparison with the Microsoft Windows... both these "methods" have their own good and bad aspects about which I won't be talking, since it's out of the scope of this post.
|The "decent" Nautilus :)...|
Anyway, why is it important to clear this Nautilus Thumbnail cache?
Well, depending on the occasion, this can come in real handy. For instance, say that after installing Ubuntu Linux for the first time, you entered into a folder which contains nothing but video files.
Say that you haven't installed the proprietary codecs, then Nautilus will fail to generate any video thumbnails and "mark" them as "failed" thumbnails. As a result, even after you install all those proprietary codecs, and whenever you enter that "failed thumbnails" folder afterwards, Nautilus, still, will not generate and display thumbnails because it now has an entry which says that it's impossible for that folder.
In that case our solution is to manually delete that cache which would resolve the matter (obviously).
So, how the heck can I do that Gayan?
Simple actually. By default Nautilus stores all those cached/thumbnail details in a folder called "thumbnails" in your "Home" folder.
So after open your "Home" folder and simply press "Ctrl" + "H" keys on your keyboard to make nautilus show hidden files... then scroll down until you see a folder called ".thumbnails" as shown in the below screenshot.
Inside that folder you'll see other 3 folders named "fail", "large", "normal". Now as you can guess, simply go inside of the "fail" folder and there will be another folder called "gnome-thumbnail-factory". Go inside that as well and simply press "Ctrl" + "A" keys and delete them all.
Now close the window and enter the folder/location which Nautilus refused to generate any thumbnails... now when you enter it... it should give you those beautiful looking picture previews, at last :D. Enjoy it!.