One force is the force of gravity, pulling downward on the box.
Now, you know that the forces acting on the box must be balanced, because if they're not, then the box would be accelerating. But it's just sitting there, so there must be some other force, just exactly the right strength and direction to exactly cancel the force of gravity on the box, so that the net force on it is zero.
The other force is the force of the table pushing upward on the box. It's called the "normal force".
