Two-step verification and devices

This is in your profile, the Security tab.

Two-step verification

1. Choose Set up two-factor authentication and enter your password.
2. Scan the QR code with an app such as Google Authenticator, 1Password or Authy.
3. Enter the 6-digit code.
4. Save your recovery codes. Each code works once and you only see them once. If you lose your phone, you sign in with those.

Turning it off asks for your password and a code.

Password

If you change your password, you are signed out on your other devices.

Devices and activity

Under Devices you see where you are signed in. Sign out a device, or choose Sign out all other devices. Under Activity you see your latest sign-in attempts and security actions.