By default, Mnemosyne uploads anonymised data related to your learning progress, for use in memory research. This data does not contain the contents of your cards, but only anonymous IDs. You can also opt-out of this process entirely by using the settings screen of the desktop application (Settings - Configure Mnemosyne - General - Storage and Data - Upload anonymous science logs.). These settings will be propagated to e.g. your phone on the next sync.