The Salt March is when Gandhi led the people to the sea to get salt from there instead of buying from the British. Gandhi rebelled using violence, against the British.
The caste system is a social structure for the way you live your life. Everything you do is based on your social class. You cannot associate with people in a higher class.
After the Salt march, England was furious because Gandhi and his followers. England decided to slaughter hundreds of Indians for trying to rebel.