Define reference database
You are trying to execute a function that requires a reference database. ChessBase needs to know which database it should use for reference purposes and will usually suggest the largest one it finds in My databases.
If that is the right one simply click OK. If you want to make another database your reference database click Browse.
Fast Switch of the Search Database for the "Reference" in the Board Window
It is possible to switch the search database for the reference display in the board window with a single click.
By doing this you can check what games have the current board position in another large database.