In case you are using 2 step verification and you want to add your account for example to iPhone or Thunderbird email client, you have to create Single sign-on password for the client.
1. Open the account settings by clicking your profile picture on the upper right corner and select settings from the opening box. You can access the same site by going to site https://www.google.com/settings/personalinfo
2. Select Security
3. From Settings, select App passwords
4. From the drop down menu, select the application and device. Then click Generate
5. Insert the 16 character password to the device or application.