Hinduism is a type of belief system. Hindu's are polytheistic. Polytheistic means believing in many gods. They also believe in reincarnation, rebirth into a better life.
Untouchable are the lowest of the low on the caste system. They have very poor living conditions. Also they have to eat other peoples scraps. Its very hard for untouchables to find work. These kind of people cant hangout with any of the upper class people.
The caste system is a social structure. The caste system tells the people where they belong in their society. You are born into your caste. Your caste determines your job, who you marry, and what laws apply to you.
Gandhi is a nationalist. A nationalist is a person that wants someone from their country to rule that country. Gandhi doesn't want Great Britain to rule India. He was the leader of the salt march. He went on this march to make their own salt so they didn't have to buy it from the British.