Short answers, pulled from the story.
The Salt March was a 387-kilometre, 24-day nonviolent march across Gujarat led by Mahatma Gandhi from the 12th of March to the 6th of April 1930 to protest the British salt monopoly. Gandhi chose salt because it was a daily necessity that touched every Indian regardless of caste, religion, or class, and the salt tax represented 8.2% of British Raj tax revenue while hitting the poorest Indians hardest.
Gandhi began the march with 78 volunteers from his ashram on the 12th of March 1930. By the time the procession reached Dandi, it had grown to at least 3 kilometres in length and more than 50,000 people had gathered at the coast. Millions more broke the salt laws across India in the weeks that followed.
After Gandhi was arrested on the night of 4-the 5th of May 1930, the march to Dharasana Salt Works continued under Sarojini Naidu. Soldiers beat the nonviolent marchers with steel-tipped lathis, fracturing skulls and breaking shoulders; no marcher raised a hand in defence. United Press correspondent Webb Miller's account of the beatings appeared in 1,350 newspapers worldwide and was read into the record of the United States Senate.
On the 23rd of April 1930, after Ghaffar Khan was arrested in Peshawar, a crowd of his nonviolent Khudai Khidmatgar activists gathered at Peshawar's Qissa Kahani Bazaar. The 2/18 battalion of the Royal Garhwal Rifles was ordered to open fire with machine guns, killing an estimated 200-250 people. Chandra Singh Garhwali and other soldiers refused the order; the entire platoon was arrested and many received life sentences.
Martin Luther King Jr. wrote that reading about Gandhi's Salt March to the Sea and his concept of satyagraha was profoundly significant, causing his scepticism about the power of love in social reform to gradually diminish. James Bevel and other American civil rights activists in the 1960s were similarly influenced by Gandhi's methods of nonviolent resistance.
The Salt Satyagraha did not produce immediate British policy concessions, and over 60,000 Indians were jailed. However, it forced the British to recognise that their control of India depended on Indian consent. Gandhi was released from prison in 1931 and met Viceroy Lord Irwin on equal terms for the first time, producing the Gandhi-Irwin Pact, and the nationwide Civil Disobedience Movement it sparked continued until 1934.