When creating a Send, you can assign one or multiple Tags to it.
Tags can be created in the Admin Panel and can have any value. You can create as many tags as needed.
You can filter the Calendar/List View based on tags.
Tags are typically used to indicate subtypes of Sends, or to indicate which Sends are done by which team. As you can filter the calendar based on tags, it's then easy to see to what extent the Sends on the calendar of one team or product are overlapping with Sends of another team.
As there is no limit on the number of Sends that can be created, you can actually use Tags to cover multiple dimensions, e.g. both the responsible team and a subtype of send.