Set how the application will log into SQL Server. The default is to use a 'Trusted Connection' which means that the web application uses Windows authentication to log into SQL Server.
Alternatively you can enter a particular user and password to perform this function. This is not recommended, particularly in a server environment as it is not secure by default. However, there are some authentication patterns that may require this, including some options for adding security. Please see the full technical articles for details.
Note that you can change this setting after the install using the 'Server Management Tool'.