What is "sql-server-localdb"

Microsoft SQL Server Express Localdb is a SQL Server Express execution mode intended for program developers. Localdb installation copies a minimum set of files needed to start the SQL Server Database Engine.

When Localdb is installed, developers initiate a connection using a special connection chain. On connection, the required SQL Server infrastructure is created and started automatically, allowing the application to use the database without complex or time-consuming configuration tasks. Developer Tools can provide developers with an SQL Server Database Engine that allows them to record and test Transact-SQL code without having to manage an entire SQL Server server instance. An instance of SQL Server Express Localdb is managed with the Sqllocaldb.exe utility. SQL Server Express Localdb should be used instead of the SQL Server Express user instance feature, which has been replaced.

Source: https://msdn.microsoft.com/pt-br/library/hh510202.aspx