You must first download the Discord mobile app and login using your account information in order to login using a QR code. Both Discord for Android and Discord for iOS support scanning QR codes.
Once you've signed in on your mobile device, you may use the app to scan a QR code to sign in to the Discord desktop or browser client.
This QR code is no longer valid because it only lasts for two minutes. Scan it to see for yourself.
Go to the Discord app and scan the login code there. Select Scan QR Code from the User Settings menu once it has been opened. If you haven't previously given camera access, you have to give camera access and then you can begin scanning. Simply align the box with the QR code displayed on the screen once the scanner has opened.