What Is a Sidecar File?
A small companion file holding information about the photo next to it. Here is why that is fragile.
A sidecar is a small file that sits next to a photo and holds information about it. Same name, different extension. Google Takeout gives you one .json sidecar per photo, holding the capture date, the location, your caption and more.
It works, but it is fragile in a specific way: the information is only correct as long as the two files stay together. Copy the photo somewhere without its sidecar and everything the sidecar knew is gone. Rename one and the pair breaks. Most photo apps do not read them at all.
Information stored inside the photo has none of those problems. It travels with the file wherever it goes.
That is what the app does: it moves what is in the sidecar into the photo itself, where every app can read it and nothing can separate them. It can also write fresh sidecars if you are importing into something that prefers them. The first 500 photos are free.