In Gmail, sync refers to the process of synchronizing emails between Gmail on the web and an email app on a mobile device. Syncing Gmail on a mobile device allows users to receive notifications and new emails automatically. When syncing is turned on, actions taken in the email app on the mobile device, such as opening a message, moving a message to a folder, or deleting a message, are synchronized back to Gmail on the web.

The sync settings in Gmail control how often the Gmail app checks for new messages, how much data it uses, and how it handles notifications. Users can choose from various options, such as Automatic (push), Every 15 minutes, Every 30 minutes, and so on. The "Days of mail to sync" setting in Gmail allows users to choose the number of days of mail that they want to automatically sync and store on their device. Any messages that are not synced will not be available offline.

Syncing Gmail with other email accounts or apps can also be done using third-party apps that support Gmail integration. To use a third-party app, users need to download it from the Google Play Store or the App Store, depending on their device, and grant the app permission to access their Gmail account.

In summary, sync in Gmail refers to the process of synchronizing emails between Gmail on the web and an email app on a mobile device. Sync settings control how often the Gmail app checks for new messages, how much data it uses, and how it handles notifications. Users can also sync Gmail with other email accounts or apps using third-party apps that support Gmail integration.