Two-step sign-in adds a 6-digit code from an authenticator app on your phone to signing in, so someone who learns your password still can't get into your account. It is off until you turn it on.
TURN IT ON: your picture (top right) > Account > Two-step sign-in > Turn on. Scan the QR code with an authenticator app (Google Authenticator, Microsoft Authenticator, 1Password, or the iPhone Passwords app), or type the key it shows, then enter the 6-digit code the app gives you. ATOM then shows 10 backup codes once: save them somewhere safe. Turning it on signs out your other devices.
SIGNING IN: after your password (or Continue with Google / Apple), ATOM asks for the current 6-digit code. Codes change every 30 seconds and each one works once. Five wrong codes and you start the sign-in again.
NO PHONE? Tap Use a backup code instead and enter one of your backup codes. Each works once; Account shows how many are left. Manage > New backup codes replaces them (the old ones stop working).
TURN IT OFF: Account > Two-step sign-in > Manage, enter a current code or a backup code, then Turn off. Your other devices are signed out.
LOST YOUR PHONE AND YOUR BACKUP CODES? ATOM support can't read your codes. Contact support to prove it's you and get it reset.
Not on the Test Drive demo.