Explanation
SQL Data Sync is a service built on Azure SQL Database that lets you synchronize the data you select bi-directionally across multiple SQL databases and SQL Server instances.
With Data Sync, you can keep data synchronized between your on-premises databases and Azure SQL databases to enable hybrid applications.
Compare Data Sync with Transactional Replication

References:
https://docs.microsoft.com/en-us/azure/sql-database/sql-database-sync-data