system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does reverse image search work?
gpt
There are 2 parts to it:

 * From each image, a "fingerprint" is extracted, something that describes the image as uniquely as possible but doesn't change when the image is manipulated (e.g. resized or changed to a different format with a lower color depth).
 * Have an index that allows you to search through millions of fingerprints efficiently to find the closest match to the fingerprint of your search image (the "efficient" part is where it gets complicated).

This is the stuff of active research, many people are trying to find newer and better algorithms for this.

A very simple fingerprint algorithm would be to resize the image to a very low resolution, e.g. 3x3 pixels with 6bits of color depth, i.e. you have 9 pixels and each can have one of 64 colors. And the search would return all images that are identical when resized to that format, which means they have a similar average color in each of the 9 parts that are sized down to 1 pixel.