what does -d in docker code example

Example 1: docker command

docker ps # current containers
docker run # create and start the container
docker create # create container
dokcer exec # to run commnads in container for once
docker volume # create a docker volume
docker network # create a docker network
docker rm # remove container 
docker images # list the images
docker rmi # remove image
docker build # build a new image from dockerfile
docker push # push your image to docker repo
docker pull # download an image from docker repo
docker commit # create an image from container

Example 2: what is docker

Docker is a set of platform as a service products that use OS-level
virtualization to deliver software in packages called containers. 
Containers are isolated from one another and bundle their own software,
libraries and configuration files; they can communicate with each other
through well-defined channels.