Google blocks Thunderbird from checking gmail if a certain setting is turned on via Google’s account security settings screen.
Some devices and apps use insecure sign-in technology to access your data.
Choosing Disable prevents these less secure devices and apps from accessing your Google Account.
Choosing Enable increases your chances of unauthorized account access but allows you to continue using these less secure devices and apps.
https://www.google.com/settings/security/lesssecureapps
Mozilla explains the problem here:
https://support.mozilla.org/en-US/kb/thunderbird-and-gmail