Usually this is used in night or space photography
In:
Imagine having a stack of physical photographs on a table. Unstack them and place them side-by-side so that the edge of one lines up with the edge of the one(s) next to it. That’s all they’re doing, just with computers.
They mean exactly that. The photo you see is comprised of several smaller photos that are stitched together in a photo editor of sorts to create a large, complete picture.
For example:
https://www.thephoblographer.com/2019/02/18/backyard-astrophotography-hits-new-heights-with-this-81mp-moon-pic/
The photo in that article is made up of 50,000 different photos.