The capacity of a container is another word for the volume of material it will hold.
It's usually measured in liters or gallons. It isn't the same as the volume the container would displace if you immersed it in water.
The difference between these two quantities is the thickness of the container walls.
This difference is negligible if the container is made from a thin material, but for wooden or concrete containers with walls that can be several inches thick, it isn't.
When measuring capacity, it's always best to measure the inside dimensions. If you don't have access to the inside, you need to know the thickness of the container walls to get an accurate result.
Hence, volume resembles the capacity of a container.