Hindus believe in reincarnation. They believe that if you're good in this life that in the next one you will be reborn as someone or something better. They have a polytheistic religion.
The salt march is when Gandhi led the people of India to the sea to make their own salt to avoid buying from the British. Gandhi rebelled using non violent tactics against the British.
The caste system is a social structure for the way you live your life. Everything you do is based on your caste. You cannot associate with people in a higher class.
After the salt march, the British were furious with Gandhi and his followers. The British decided to slaughter hundreds of Indians for trying to rebel.