Samudra manthan: Churning of the ocean, a symbolism
Samudra manthan or Churning of the Ocean of Milk (aka Sagar manthan, Kshirsagar manthan) is one of the famous stories in Hindu Mythology.
According to the legend: Indra was cursed by the sage Durvasa for his arrogance, following that the Suras (Gods: Sattvic) lost the battle to Asuras (demons: Tamasic). To regain the immortality with the help of nectar of immortality, the Suras formed an alliance with the Asuras and churned the ocean on the term that the outcomes of the churning will be equally shared among them.
Mandara Mountain was used for churning shaft, Vasuki the sepent was used as rope and Vishnu became a giant toroise and supported the mountain on his back.
Many treasures (Ratnas) came out of the ocean. Which were shared amongst the Demons, demigods and Vishnu.
As a Symbolism of the spiritual churning within each person
The Evil side and Good side within us are always pulling the Kundalini energy (symbolized by serpent vasuki) up and down on our spinal cord (mandara parvata). When the Good side seeks the help of the Lord (Vishnu) he agrees to help the devotee by supporting this battle (churning).
When a devotee is fighting a hard battle with evil forces, bad qualities (poison) like Anger, lust, doubt, suffering torment him. Again the Lord/ Guru (Shiva) comes to the rescue, he takes his devotee's burden/ Karma.
After a long time when the Good side starts inching ahead of the evil side many Siddhis (treasures) come out, When the Good side keeps winning without giving up and finally wins the devotee (Suras) gets liberation, deathlessness (the Nectar of immortality)