I believe what they mean is to create some kind of uniformity in your thumbnails, I started doing this not to long ago and personally it has made my channel look much nicer and appealing. So basically I have a background that I always use, I always add in this avatar in the corner, and then I have a design that covers one of the top corners and one of the bottom. Always use the same font (not necessarily colour but if you have a font that is on your banner then i would suggest that. Once you have this made you can then add in things like video specific and images (if that is the style you are going for).
If you would like my example of this then look at the thumbnails on my channel and some of my featured channels.
I know that this is coming from the gaming perspective but I hope that it gives you some idea. Great channel btw.