App passwords or app-specific passwords are randomly generated passwords for authentication. App passwords are required for accounts with two-factor authorization (2FA). Providers also require app passwords for third-party access, such as Nylas.
An app password is required if 2FA is enabled and authenticating without a security code.
Yahoo Mail accounts require an app password for Nylas to authenticate. An app password gives Nylas access to the account through the single password login.
Apple ID uses an app password for accessing information in iCloud.