A stack is a collection of services, usually related to one application or usage. For example, a WordPress stack definition may include a web server container (such as nginx) and a database container (such as MySQL).
When the new image notification feature is enabled, the Images up to date column indicates whether the local images in the stack are up to date, with a green circle indicating they are up to date and a red circle indicating that there is a newer version of an image available at the remote registry. A grey circle indicates Portainer was unable to determine whether there is an update available for the images.
For more on how this works, have a look at this knowledge base article.