“Dual density” refers to a specific way that a toy was constructed with the material (silicone is most common but it’s also available in other materials) so that the inner core of the shaft is firm enough for penetration and to withhold its shape, however the outer layer of the material (exterior) is softer, almost squishy. These types of toys provide a way to get the best of both worlds, a toy that feels softer and more flexible, but sturdy enough for comfortable penetration!