A datasource represent a database connection. Datasources can be viewed on the datasources screen
The Add Datasource button can be used to create a new datasource. The add datasource form requires you to enter the datasource name and the database connectiond details
Adding datasource to a backend
The add to backend button under the Options column can be used to add a datasource to a Backend [link to Backend doc].
Once added, the datasource is visible in the SQL Manager [link to Sql manager docs - need doc for this].
The SQL manager will use the selected datasource to perform SQL queries on and to create REST resources. [link this to the add to resource section in the SQL manager doc]