Ex opere operato is a Latin expression meaning "by the work worked." It refers to the fact that the sacraments confer grace when the sign is validly effected -- not as the result of activity on the part of the recipent but by the power and promise of God.
Now, to receive the fruits of the sacraments, you should be properly disposed. At least in adults, there must be a predispositional receptivity to receive the grace that is always available in a validly effected sacrament. This means reception of grace via the sacraments is not automatic. But the ex opere operato nature of the sacraments reminds us that, while a proper disposition is necessary to receive grace in the sacraments, it isn't the cause of that grace.