redis-server is the name of the container that is created by the command and redis is the image from which the container is built.
docker run -it --link redis-server:redis redis redis-cli -h redis
means a container from the image ‘redis’ will be created, and the command ‘redis-cli -h redis’ will be executed on it. The tutorial probably wants to tell you that ‘-h “name of other container”’ works because of the linking.
The names have been chosen a bit confusing by the creaters of that tutorial.