Devi Durga mantra
The most chanted mantra of Maa Durga is following. The meaning of this mantra is as following.
Sarva-Mangala-Maangalye Shive Sarvaatha-Saadhike I
Sharanye Trayambake Gauri Narayani Namostute II
We bow to you O Devi Narayani. You are the the auspicious for everyone, you bring happiness . You are the one who completes every work for everyone. You are protecter of everyone.
You are the one with 3 eyes and a beautiful face; We salute you, O Narayani.
Meaning of Durga Mantra
She is the most auspicious one and the one who bestows auspiciousness upon all the worlds. She is pure and holy.
She protects those who surrender to her and is also called the three eyed one and Gauri (daughter of mountain king). I bow down to Mother Durga who is the sister of Mahavishnu.
Goddess Durga manifested to kill the demon Mahishasura or the buffalo faced demon. Till then, the demon Mahishasura appeared invincible and was troubling all the worlds beyond compare. The heavens and the worlds suffered heavily in his evil hands. The gods and humans were praying fervently to Brahma, Vishnu and Shiva seeking to put an end to the miseries they suffered in the hands of the demon. Time passed with no hopes. It was then that goddess Durga manifested in response to the prayers of all.