Durga means
Srimad-Bhagavatam
SB Canto 8
Prahlāda Mahārāja said: O Supreme Personality of Godhead, You are universally worshiped; even Lord Brahmā and Lord Śiva worship Your lotus feet. Yet although You are such a great personality, You have kindly promised to protect us, the demons. I think that such kindness has never been achieved even by Lord Brahmā, Lord Śiva or the goddess of fortune, Lakṣmī, what to speak of other demigods or common people.
The word durga-pāla is significant. The word durga means "that which does not go very easily." Generally durga refers to a fort, which one cannot very easily enter. Another meaning of durga is "difficulty." Because the Supreme Personality of Godhead promised to protect Bali Mahārāja and his associates from all dangers, He is addressed here as durga-pāla, the Lord who gives protection from all miserable conditions.