To "publish" means that changes made in the web interface are sent to and carried out on players. Players then have to react to this information, downloading new content if needed.
Publishes are done from the Overview section in Portal. If a change has been made that needs to be published, you will see a small red symbol next to the Overview icon, and the text "X unpublished changes" in the Overview page.
To publish, click the arrow next to X unpublished changes, either select Publish all (which will publish any currently visible changes),
or click on the padlock to manually select which changes to publish by checking the box next to it and then click the ✓ button.
The icon to the left indicates what type of item it involves (player, playlist, etc).
It is possible to set the network to Auto publish meaning that it will publish any changes immediately.
This setting is made in the Control Panel.
You can also click the Tools icon (1) and then Publish later (2) to open a dialog (3) where a later publish can be set up.
The publish later applies to all items in the publish list.