Idk what that is, nor do I care at this time considering Strat was nice enough to aware me about it.
You know search engines? Where if you search for "cats" it'll show you images of cats and sites that have cats on them? Reverse image searching is where you get an image and make the search engine look for sites which have that image on it. Tineye, Google Images, IQDB and SauceNAO all do this ^^ It can be incredibly handy for finding larger versions of images, like if you know there's a wallpaper and the image you have isn't the right resolution, or finding the source if you want more from an artist!
#1 was originally made by Pixiv user dingyiyi, and you can view the image here (or here if you don't have a Pixiv account!)