In Microsoft Teams, a guest is a person who is not an employee of your organization. A guest doesn't have a school or work account with your organization. Your Microsoft Teams administrator should turn ON the Guest Access in order to allow you to communicate with people outside your organization.
As communicated in December 2020, Microsoft is now making some changes to the default setting for Guests in Microsoft Teams. If you have not already configured Guest access capability in Microsoft Teams, that capability will be enabled in your tenant by default.