What is Authentication?
Authentication is the system that checks "who is this user?" On a website, things like signing up for an account, logging in, logging out, and showing only the logged-in user's own data are all handled through Authentication.
- Signup — creating an account for a new user
- Login — signing in with an existing account
- Auth state — letting the whole app know whether a user is logged in or not
- Logout — closing out the session
Real-world usage
If you want a Notes App to show each user only their own notes, you can't do that without Authentication. You can use the logged-in user's uid to keep their data separated in Firestore.