There are two login methods that you can use for logging in to the Steam launcher.
The first one is the default one, which uses a nickname and password. After you enter your credentials, press the "Sign in" button to log in to your Steam account. If the Steam Guard option is available, you'll need to additionally confirm the login procedure.
Note: Please make sure that you are using the right language during the login procedure. You can change the remote gaming desktop language in its overlay if you are using the browser version of the service. If you are using our native desktop application, the language of your system will be used during the login procedure
The other method uses the "Scan a QR code" option which is only available after configuring the Steam Guard Mobile Authenticator. If this condition is met, please follow these steps to log in to your Steam account:
Open the Steam Mobile App and log into your Steam account.
Go to the Steam Guard menu (the middle "shield" icon in the bottom navigation bar) for the "Scan a QR code" prompt.
Hold your phone up to the Steam sign-in panel and use the mobile app to scan the QR code.
You can also check this video for visual reference regarding these login methods: