mountPath shows where the referenced volume should be mounted in the container. For instance, if you mount a volume to mountPath: /a/b/c, the volume will be available to the container under the directory /a/b/c.
Mounting a volume will make all of the volume available under mountPath. If you need to mount only part of the volume, such as a single file in a volume, you use subPath to specify the part that must be mounted. For instance, mountPath: /a/b/c, subPath: d will make whatever d is in the mounted volume under directory /a/b/c