A flag is an indicator of some chess characteristic that can be turned on or off for each game in the database. There are 13 user-settable flags that you can directly set for each game. Of these, only the Delete flag has any special significance: games with the Delete flag turned on are marked for deletion and will removed when the database is compacted.
The other 12 user-settable flags and their symbols are:
A flag can be set for the current game, all filter games, or all database games using the maintenance window.
You can use a header search to find all games in a database that have a particular flag turned on or off, or use flags as part of more complex searches.
Since all the user-settable flags (except the Delete flag) have no significance to ChessDB, you can use them for any purpose that suits your needs. For example, you could use the Kingside (K) flag for kingside pawn storms, or kingside heavy piece attacks, or even for endgames with all pawns on the kingside.