Put simply, a container is a runnable instance of an image. Containers do not hold any persistent data and therefore can be destroyed and recreated as needed.
When the new image notification feature is enabled, the circle next to the image name indicates whether the local image is up to date, with a green circle indicating it is up to date and a red circle indicating that there is a newer version of the image available at the remote registry. A grey circle indicates Portainer was unable to determine whether there is an update available for the image.
For more on how this works, have a look at this knowledge base article.
To add a new container, click Add container.
Once a container has been created you can inspect it, edit or duplicate it, toggle a container webhook, attach volumes, view logs and statistics, edit ownership, and access its console.
If you no longer need a container, you can remove it.