A tag is short piece of text that you wish to associate with a request.
A request can have multiple tags associated with it.
Tags can be added and changed using the Manage History Tags dialog.
They can also be automatically added via Passive scanning - this allows you to automatically tag pages which match a particular regular expression.

Tags are shown in the History tab and can be filtered on using the History Filter dialog.

See also

UI Overview for an overview of the user interface
Features provided by ZAP