Stretch Database is a new feature in SQL Server 2016 that will allow to migrate old data from on-premises to Microsoft Azure and still provide the ability to access the data without making any query changes. Stretch Database provides the advantage of the lower costs of Azure Database and reduces the need to buy hardware and storage to support on-premise data archiving. Enterprise Storage is not always cheap.
Stretch Database is ideal for entire tables or part of a table. e.g. Orders that are older than a certain date or Customers who haven't made any orders for more than a year etc.
Stretch Database transfers data securely using Always Encrypted, so data is encrypted as it is being transferred from on-premises to Azure.
Note: Stretch Database only works between an on-premises SQL Server Database and Azure SQL Database, and cannot be used from one local SQL Server database to another.