A Team is a group of Agents who are working together at the owning organisation. Depending on the structure of your organisation you may prefer to have a team of staff responsible for actioning tickets rather than each Agent individually. This is a good way to structure your Helpdesk in order that tickets get assigned to the correct Team/department as soon as possible rather than sitting unassigned in a queue. Your Sales team for example would be answering very different tickets to your support team so it would be beneficial to set them up in different Teams.
An Agent may optionally be in multiple Teams. This is practical for management and team leaders in order that they can answer their own tickets but also have visibility of their staff within another Team. There are various settings and options which will dictate how each Team is assigned tickets and how they operate. More on this in Managing Teams.
Teams settings can be found under Organisations & People as below.