What does the error to sign in with an app-specific password in Transporter mean?

App-specific passwords are required if your Apple ID uses two-step verification. If you have two-step verification enabled, you may see these two notifications:

Please sign in with an app-specific password. You can create one at appleid.apple.com (-22910)

• Your Apple ID is enabled for two-step verification, and you'll need to generate an app-specific password at appleid.apple.com. After you set that password, you'll have to use it whenever you sign in to Transporter.

Sign in with the app-specific password you generated. If you forgot the app-specific password or need to create a new one, go to appleid.apple.com (-22938)

• Your Apple ID is enabled for two-step verification and you've already generated an app-specific password for Transporter. Use that password to sign in to Transporter. If you forgot the password, create a new one at appleid.apple.com.