What's a MS SQL "named instance" ?

When installing SQL Server on Windows Server (like on EC2) you have the option to "name" the service running the SQL Server Database Engine. You can install multiple copies of SQL Server Database Engine side-by-side by providing different names. This is often used in clusters to optimize the use of hardware. When connecting to SQL Server you need to then specify both the host name as well as the service name in the connection string, like

Server=myServerName\myInstanceName;Database=myDataBase;User Id=myUsername;Password=myPassword;

RDS does not provide this level of control and will always use the default service name internally. So you would have to connect using the just the hostname and database name, like

Server=myServerName;Database=myDataBase;User Id=myUsername;Password=myPassword;