For example, Jane measures the box. When she measures the distance between the end of the box closest to her and the end of the box farthest away, Jane measures the box's depth.
For liquids, the distance between the top or surface of the liquid and the bottom of the liquid is the liquid's depth.
For example, water is the liquid. If you fill the container with water, the distance between the top of the water and the bottom of the container is the water's depth. If the depth is big we say the water is deep.