That is the name of your container. You can go ahead and name your container using the '--name' flag in your container run command. Since you did not give a name for the container docker generated a random name one
Example of naming a container:
$ sudo docker run --name my_container ubuntu
the above command should startup an ubuntu container with the name my_container.
Naming containers is important for establish links, Docker relies on the names of your containers to make any sort of connections between each other.
To get to know more about how docker containers and the linking between them works check out docker training course.