This is only available for Tribe community admins.
In Admin Panel> Settings> Messages you can define Moto, Signup rules, Ask rules, Question intro, User intro, Topic intro, and User Terms and Conditions.
User Terms and Conditions is a message that appears on the sign-up page of the community. By adding a message to this section, a checkbox will be added to the sign-up page and by clicking on the checkbox, your members are agreed to your Terms and Conditions (T&Cs).
You can put a message like "By clicking on the checkbox, you are agreed to our T&Cs". You can attach a link of your T&Cs page to the message as well, so users can click on it and read it before clicking on the checkbox.
Please be advised that this option allows you to add the T&Cs to the Tribe community registration page. If you are using SSO, this option is not applicable. As an explanation, when you are using SSO, basically, you are redirecting users to your main website's login sign-up page. Tribe does not have control over those external pages. Therefore, if you want to add anything to this page, you need to handle if from your website and not from the community settings.