The labels on campaigns can be used to group campaigns together which can then be used to mass build and publish campaigns (How to use the Mass build and Publish Campaigns feature) having the same labels.
To create a new label
Labels are created at template level.
Navigate to campaign templates in the config section of the catalog.
Click on campaign templates and scroll to the label attribute and click on the dropdown in the right.
Screenshot: Label attribute for campaign template
New label can be added use the Add (+) option at the bottom. Click on Save once the label is created.
Screenshot: Creating new labels
To add label on a campaign
Navigate to the attributes of the campaign.
Screenshot: Navigating to attributes of a campaign
Scroll to the bottom to find the Label option.
Select the label to apply and click on save.
Screenshot: Adding labels to a campaign
To remove the labels
You can remove the labels by clicking on the cross (x) mark next to the labels.
When there is only one label you can remove it by selecting the label again which deselects it.
Screenshot: Removing labels from the campaign