Duo is a service you use for multi-factor authentication. Register a device with Duo, and then choose one or more methods by which to authenticate. You can:

• Receive a push notification on your mobile device
• Generate a passcode with a YubiKey
• Receive a text message
• Generate temporary passcodes
• Receive a phone call

When you use one of the methods above, you’re given access to the requested online service.