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 indicator feature is enabled, the Images up to date column indicates whether the local images in the stack are up to date, with a green tick indicating they are up to date and an orange cross indicating that there is a newer version of an image available at the remote registry. A grey hyphen indicates Portainer was unable to determine whether there is an update available for the images.
You can click the Reload image indicators button to recheck the images for your stacks for updates, or to recheck a single stack's images you can click the image indicator icon for that stack.
For more on how this works, have a look at this knowledge base article.
