Hey there!
The untouchables were people who were looked down by the higher castes in India. They were the people belonging to the lower castes who did all the minimal work. They were called untouchables as the higher castes thought that touching them would make them impure. Mr. Gandhi (often referred to as the father of the nation in India) used to call them as 'Harijans' which translates to 'children of God'. Now, they are known as the dalits.
Hope it helps ya!