Verifying a user's identity before letting them in, using a password, a one-time code or another account.
Authentication answers the question who are you? It can use email and password, a code sent to a phone, sign-in with a Google or Apple account, or biometrics on a phone. Once verified, the user gets a session so they can use the app without signing in on every page.
Sign-in is one of the most security-sensitive parts of an app and one of the most important for user experience. Complicated steps lose customers, while weak protection exposes accounts to takeover. In the region, phone number sign-in with an OTP is common and familiar to customers. Practical tip: do not build authentication from scratch; use a proven solution or managed service. Never store passwords as plain text, and limit failed login attempts to block guessing.