What does synch all accounts mean?

Synchronisation is the process that keeps Thunderbird for Android's mailbox view aligned with the server. It checks for mailbox changes, retrieves new emails and sends outbound ones. The app runs this process (polling) typically every 15 minutes. If you click the 'Sync all accounts' button, it will check for new emails on each account immediately rather that waiting till the next 15 minute interval is reached.