An incredibly generic answer is that metadata is just “data about data”. In the given example, the photo was the data, so time taken, location, phone model, etc is all data about the picture, i.e. metadata. The same can be true of any kind of data. If you’re used to windows file extensions you could even think of those as metadata. “.txt” doesn’t change the contents of a file, but it does tell you that it’s text.

https://xkcd.com/356/

You have to disable it from the proton side

https://proton.me/support/automatic-forwarding-gmail#how-to-stop-automatic-gmail-forwarding