Short answers, pulled from the story.
A salt tax is a direct levy on salt, usually proportional to the volume purchased. The practice dates to at least 800 BC, when China levied the first known salt tax, though salt itself was used in religious offerings and trade as far back as 6050 BC.
The Gabelle was the French salt tax, first implemented in 1360 and lasting, with brief interruptions, until 1945. It was structured by social class, placing the heaviest burden on small farmers and poor urban residents, and is considered a contributing factor to the French Revolution.
Gandhi protested because the British Salt Act of 1882 prohibited Indians from collecting or selling salt, forcing them to buy heavily taxed British salt. In 1930, he led a 24-day, 240-mile march from Sabarmati Ashram to the coastal village of Dandi to illegally harvest salt from the sea as an act of civil disobedience.
The Moscow Salt Riot began when the Russian government replaced several existing taxes with a universal salt tax to replenish the state treasury. The price of salt rose sharply, and the riot was intensified by widespread elite tax evasion that left the poorest residents bearing the full cost.
The Salt Tax Revolt took place in the Spanish province of Biscay between 1631 and 1634, in response to Philip IV's taxation policy over the price and ownership of salt. The revolt broadened into a protest against general economic inequality and ended in spring 1634 when the main leaders were executed; Philip IV nonetheless revoked his original salt orders.
England abolished its salt tax in 1825. The repeal was driven largely by industrial manufacturers who needed cheap salt to produce sodium carbonate through the Leblanc process, making the tax an obstacle to industrial production rather than just a hardship for consumers.