The following link will provide basic idea about the @@TRNANCOUNT variable in SQL Server. As per MSDN,
@@TRNANCOUNT will
Returns the number of BEGIN TRANSACTION statements that have occurred on the current connection.
For further details about how it's value us incremented/decremented, refer to the following link:
https://msdn.microsoft.com/en-in/library/ms187967.aspx
Happy Querying...!!!