What was the Dandi march?

In 1930, Mohandas Gandhi led his followers on a 24-day march to the sea to gather salt, an act that had been illegal since 1882. In 1882, the British had imposed a monopoly on salt, making it a criminal activity to gather salt. Instead Indians were forced to buy it from the government and pay a tax on the purchase.  
The Dandi or salt march was the first act of Satyagraha or nonviolent resistance by the Indian National Congress against the power of British raj, which ruled India with an iron fist. At the very end of 1929, the Indian National Congress declared independence from British rule and demanded Indian sovereignty over their own country. 
Satyagraha or nonviolent truth force, sometimes understood as speaking truth to power, was integral to the campaign for Indian independence. Gandhi insisted that the actions taken to gain independence had to be "pure" or self rule could never have lasting success. He spent a great deal of time training his followers in the principles and discipline of non-violent protest. In fact, he was so worried about violence that he took his initial 80 marchers from his ashram so that he could be sure they properly understood Satyagraha.
Although initially ridiculed by some, Gandhi had good reasons to launch the campaign for independence with the Salt March:
The tax was a concrete injustice that everyone understood. Why should Indians have to pay a tax to the British on a universally needed item that was freely available by evaporating salt water?
The tax disproportionally hurt the poorest Indians. It was thus a potent symbol of how the British exploited India.
Gandhi hoped protesting the tax would rally Muslims as well as Hindus to the cause, as everyone paid the tax.
Gandhi knew he had to protest something very concrete, specific and easy to understand if he was going to successfully rally people to shaking off British rule. He believed that taxing salt was similar to taxing air: an unfair tax on a basic necessity of life. 
The march did not gain concessions from the British. However, it was successful in swaying world opinion in favor of the Indians. Many papers followed the march and newsreels of Gandhi taking salt from the sea were widely distributed. Further, it led to mass civil disobedience as Indians across the country made their own salt.

it is also known as the dandi march or the salt satyagraha