What do the flags in docker run -d -p 8080:80 --name web nginx do?

The command starts a container from the nginx image and runs it in the background with a published port and a fixed name.

• -d runs the container detached, in the background, and prints its id. Without it, the container runs in the foreground and holds your terminal.
• -p 8080:80 publishes a port in the form host:container. Traffic to port 8080 on the host is forwarded to port 80 inside the container, where nginx listens.
• --name web gives the container a readable name, instead of a random one, so you can run docker logs web or docker stop web.
• nginx is the image. Without a tag it means nginx:latest, and Docker pulls it first if it is not local.

docker run -d -p 8080:80 --name web nginx
curl http://localhost:8080
docker logs web
docker rm -f web

By default the port is bound on all interfaces. Use -p 127.0.0.1:8080:80 to expose it only to the local machine.

The left number is always the host, and the right one is the container.