A one-sided dame is a dame that can only be filled by one player. Filling it yields one point under area rules, such as Chinese Rules, Ing Rules, and AGA rules, and under certain territory rules, such as Lasker-Maas Rules and Spight Rules. Doing so does not gain a point under Japanese Rules or Korean Rules.
This position is seki. White can never make another move here, as a move at any of the three shared liberties will result in the death of his group. Black, however, can make one more move here (but not two). Under area scoring, that move will count as a point (as stones are points). As black can wait with playing this move until all the normal dame are filled, he gets an extra point more than under Japanese or Korean rules.