You know Dropbox? Google drive? OneDrive? That's file synchronisation. Files across multiple devices kept in sync by the software provider. Except in the named cases above, all your data is uploaded to their servers. With syncthing there's no cloud server, just your devices operating over the internet. So you have some backup responsibility to cover.

Caveat: I've never used syncthing and I wrote the above with a total of 10 seconds of reading their website and so it is entirely possible I'm completely wrong about everything and so I emplore you to do your research.