Why is Maa Durga called Mahishasuramardini?

The story goes that the demon Mahishasura had gained the boon of immortality from Lord Brahma that no man or God could kill him. He then started harassing the three worlds. The demigods raced to the Trimurti -- Brahma, Vishnu and Mahesh -- who combined their energies to manifest the Goddess Durga and gifted her their individual weapons to slay Mahishasura.

Metaphorically, Mahishasura represents the tamas, or sloth and the other shadow aspects that all of us have to deal with and overcome to make spiritual progress in life.