The SQL Server Stretch Database is a special variant of the SQL Database for hybrid scenarios and comes into action when you need to store large amounts of transaction data from an on-premises SQL Server Database (starting with SQL Server 2016) over a long period of time (so-called cold storage) in the cloud.
As soon as you have activated the stretch functionality in your SQL Server, the data released for archiving will automatically be transferred to an Azure SQL Database.
For more information, visit https://docs.microsoft.com/en-us/sql/sql-server/stretch-database/stretch-database.