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.
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