It’s volume driver, equivalent to
docker volume create --driver local --name esdata1
docker volume create --driver local --name esdata2
local means the volumes esdata1 and esdata2 are created on the same Docker host where you run your container.
By using other Volume plugins, e.g.,
--driver=flocker
you are able to create a volume on a external host and mount it to the local host, say, /data-path. So, when your container writes to /data-path, it actually writes to a external disk via network.
Refer here for some sort of Volume plugins available