The British introduced a law stating that the Government had control over the manufacture and sale of salt and also imposed a tax on the sale of salt. Gandhiji and other leaders felt that it was unfair to impose such rules on essential food commodities like salt. In 1930 Gandhiji led a march to break the salt law. Gandhiji and his followers marched over 240 miles from Sabarmati Aashram to Dandi(on the west coast of Gujarat) where they broke the Salt law by gathering natural salt found on the shore and boiled sea water to produce salt. 
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Salt  was  a  necessary  thing  for  the  people  and  due to  the  law  people  were  not  allowed to  make  salt on  their  own.They  had  to  buy  it  from  the  British  government.They  had  to  pay  tax  for  purchasing  salt.So  gandhiji  started  the  non-violent  and  tax resistant  Salt  March.