This is an array which returns a reference to the specified DB Connection link object, allowing you to then use the properties of the DB Connection object.
DB Connections are a zero (0) based array, therefore zero (0) is the first indexed DB Connection, one (1) the second, and so forth.
To access the actual DB Connection object, you need to use the .DBCon property of the link object, as shown below. More information can be found at DB Connection Class (DBCon).