Containers are like sandboxes for the applications running inside them. This is mostly beneficial and wanted in order to protect applications running in different containers from each other. That also means that the whole filesystem visible to an application running inside a container is private to this application and no other application running in a different container can interfere with it.
At times though, we want to share data between containers. Say an application running in container A produces some data that will be consumed by another application running in container B. How can we achieve this? Well I'm sure you've already guessed it—we can use Docker volumes for this purpose. We can create a volume and mount it to container A as well as to container B. In this way, both applications A and B have access to the same data.
Now, as always when multiple applications or processes concurrently access data, we have to be very careful to avoid inconsistencies...