Two-Factor Authentication

Two-factor authentication is the most effective way to protect your org’s user accounts. As a Salesforce admin, amplify your org’s security by requiring a second level of authentication for every user login. You can also require two-factor authentication when a user meets certain criteria, such as attempting to view reports or access a connected app.

Secure Your Users' Identity

Refer Trailhead Module for Complete Step by Step Procedure

https://trailhead.salesforce.com/en/content/learn/modules/identity_login/identity_login_2fa

Salesforce Login

Open Salesforce Authentication App, click the Approve Button to finish logging in to Salesforce as User.

After the User logged in this screen will appear.

If the Deny button is clicked below screen will appear.

Thanks

Amol Gawali