This is a very simple use of SQL Server global variables. Microsoft SQL Server provides a massive number of global variables, which are very effective to use in our regular Transact-SQL. Global variables represent a special type of variable. The server always maintain the values of these variables. All the global variables represent information specific to the server or a current user session.
Global variable names begin with a @@ prefix. You do not need to declare them, since the server constantly maintains them. They are system-defined functions and you cannot declare them.
The following transact-sql can be use for to Get the Server and Current database as well.
SELECT 'My Server is: ' + @@servername AS DBServer
SELECT 'My Current Database is: ' + db_name() AS CurrentDataBase
For more information can be found at this link.