Kati was a sage. His great grandson was Katyayan. His daughter was born from from the blessings of Brahma , Vishnu and Mahesh. She is Katyayani. When she was born all the Gods came down to give her many many unique gifts – shankh chakra etc
There was another king named Rambh He was in love with Mahishi. Now due to a certain curse – she became a water Buffalo. Their son was called Mahisasur. Mahisa means water Buffalo
Rambh had a brother Karambh. Both brothers did penance. Karambh prayed to Varuna ( water God) and Rambh did it to Agni. Karambh was killed by Indra – who became a crocodile and killed Indra
Mahisasur became powerful when during the cremation of Rambh, his mother got burnt in the fire so he jumped in and saved his mother
She killed Mahisasur and is also known as Mahisasurnardini (mardini – destroyer/ killer)
A more detailed account can be found here
Short version – awakening of shakti and kills sex desire
Now this is a curious thing. Most people worship for things they want in this wrold. They want more money, fun, joy,and of course sex, food etc
How many people worship God, asking God to take away the thing that brings them back again and again – the desires for food and sex?
If you truly want to be a Yogi, be only with God, can you ask God, to take away all your sexual desires and food, etc?
So being a yogi means you recognize that your desires, all desires for the flesh are binding. They are your enemies. The worst enemies. Hence in hinduism they are called the demons. Sadly, we sleep with the demons, encourage the demons and pursue life for fulfillment of desires.
A more exciting detailed interpretation to follow