The Durga Sukta is one of the oldest Vedic texts associated with the concept of Durga. The word 'durga' here means 'difficult to cross' or 'inaccessible' — referring to the hard passages and obstacles of life. The prayer is: may Agni carry us across all such durgani, like a boat across a river.
This is striking: in this Vedic context, Durga is not yet the ten-armed goddess of the Puranas. She is the principle of crossing over — being ferried safely through life's most treacherous passages. Fire (Agni) is the divine ferryman.
Over centuries, this concept of 'the one who takes us across the inaccessible' (durgati nashini — destroyer of difficulty) became personified as the Goddess Durga. The Puranic Durga — fierce, armed, the slayer of demons — is the evolved form of this same ancient prayer for safe passage.