Yandex is a Russian search engine which also provides personal services like email, address books and calendars.
If you have a @yandex.ru account, choose "Login with email address" and enter your Yandex email address and your password.
If you're using Yandex with a custom domain, choose "Login with Base URL":
Base URL: https://yandex.ru
User name: your Yandex email address (email@example.com)
Password: your Yandex password
Contact group method: groups are per-contact categories. Only the group "Phone contacts" in the Yandex address book will be used for CardDAV (i.e. be synchronized with DAVdroid)! You can assign contacts to other groups and "Phone contacts".
You might need to use your app specific passwords to connect your Yandex account with DAVdroid. This always applies if you have activated two factor authentication for Yandex.