I don’t know what I should enter into the “Tag” field when I add the campaign. What is the tag used for?

The tag is not required but helpful. The tag will be added before the campaign name in the list of campaigns. The tag is a word or phrase that allows you quickly find the campaign in the list.

All the tags you use will be listed on the left side next to the list of your campaigns in the account. If you click on a tag, you will have only the campaigns with that tag listed on the screen. It’s a kind of a filter for campaigns.