Authentication allows you to identify users and restrict what data they can see and edit. Your product will need a way for users to sign up, log in, log out and, if passwords are used, a way to recover access after losing a password. This is a complicated feature set to implement, especially given that any bugs impact security. You can implement all of this using Convex, or better yet leverage a Convex integration with an existing third-party auth provider such as Auth0, Clerk, or Lucia.
Screen grab of Authentication in action