The MEX app is primarily online, this means you will have a local copy of selected data and documents on your device. Depending on the sync option you have chosen (automatic or manual), this data is synchronised back to the server to be shared with everyone.
When a sync is initiated, the mobile device will first scan its synced records for anything that has changed. Once this is complete the app updates the server and any records from the server that have also been updated since your last sync will also be downloaded to the device.
For more information about this process, go to Understanding MEX Mobile syncing.