system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 : how does reverse image search work? 

How boys and girls?  How?
gpt
well it basically works like google.
First, you'd want to know where are **every** image on the internet. So you'd need 'robots' (really programs) that scan the internet all day long for images. (You can just use google's but whatever).
Then you want to compare images? But how to?
The "easier" way would be to compare pixel by pixel each image, and to change as soon as you suppose the two images to be too different. But that would be extremely long. So the best is to do a "map" of frequencies you found in the image.
Effectively, each image could be depicted by different mathematical factors  linked with the presence of different frequencies. A low frequency represents the background ( similar colors, with a few changes between pixels) and a high frequency represents the details of the picture (edges and such.)

So basically, if you depict a map of the frequencies in the images, you could just compare a few of the frequencies factor, which are less variable than the color, which doesn't vary that much when the size of the image is changed, and which are mmmuuuuuuuch less heavy than an actual image file .
FYI That's actually also the way algorithms like shazam's works...