Create an App Specific Password to sign in to your account securely when you use third-party apps with your Apple ID.
What is an App Specific Password?
Well, App-specific passwords make’s sure that your Apple ID is not compromised connect your iCloud account to a third-party email, calendar, or contact manager app. It creates a password that those third-party apps will store instead. It a great layer of protection that will ensure you’re protected from hacker and more.
You must have two factor authentication enabled as this is crucially for this feature to work.
How To Create an App-Specific Password
Step 1: Go over to appleid.apple.com from your web browser and sign in with your Apple ID and Password.
Step 2: Make sure you verify your identity with two-factor authentication.
Step 3: Under the Security section, select Generate Passwords.
Step 4 : Enter a label for the password and click create, this can be anything, just make sure its relevant to what you are using the password for.
Step 5: Done! You have created your own App Specific Password.
Step 6: Launch the app for which you need the app-specific password.
Step 7: Paste the app-specific password into the password field that is asking for your iCloud password.
From now on, the app you just generated the app-specific password for will use this specific password in order to access iCloud.
Revoke & View App-Specific Passwords
To manage your app specific passwords expand the Security section and click the “View history” Button. You will now be able to manage your passwords from here.
At any given time, you can have up to 25 active app-specific passwords. If you need to, you can revoke passwords individually or all at once.
For example, use app-specific passwords with Microsoft Outlook, Mozilla Thunderbird, or other mail, contacts, and calendar services not provided by Apple.
If you want to find out some more information check out this link.