What hides inside a photo
Phones and cameras save extra details into every photo: the exact GPS location, the date and time, the device model and sometimes its serial number. Anyone who gets the original file can read them.
How the cleaner works
For JPEG and PNG it removes the metadata blocks directly, so the picture itself isn't re-compressed and keeps its full quality. The orientation flag is kept so photos don't turn sideways. Other formats are re-saved without metadata.
Questions
Are my photos uploaded?
No. Everything happens in your browser.
Don't social networks remove this anyway?
Many do, but email, chat apps that send "original quality", cloud links and forums often keep it.