Basically like a word search, but, for images. Google, for example, can find other references or webpages with the same or very similar images. Another website I know of is called TinEye.

Some more info on Google's image search here:
[support.google.com]

You can download an app for free.

Simply add a photo from an image you have stored in your gallery and press search. (You might need to crop it first. Which is not difficult.)

What happens next, is the software searches the Internet for matching images. Sometimes the original source of the image can be found.

It sounds much harder than it is. I've made mistakes, but that's the best way to learn.