Xu (1)虚: Empty.
Shi (2) 实: Solid.
One is empty while the other one must be solid. Taijiquan is the art of Empty rooted in the Solid and vice versa.
Empty and Solid must be equal. Use the feet as an example. One cannot stand on one foot and then the other. One must be on both feet at all times. But one foot is used to work while the other is used to support.
This is the overall way of action in Taijiquan.