Hindus believe that your caste could be changed by the law of karma.
Hindus have a Caste system, it is a system where people are grouped according to their contribution to the society. Society is divided into four main groups according to their importance (Brahmin, Kshatriyas, Vaisyas, Shudras) and those not included in the four main groups are called untouchables.
Hindus also believe in Reincarnation wherein the physical body may die and wither but the soul can live again through a different form. They also believe in Karma where your actions and its consequences will affect you over time.
Because of these beliefs, Hindus believe that if you are borne in a caste where you prefer to belong to a higher caste, you must do good in life and accumulate good karma so that when you are reincarnated, you will be able to live a life under the caste you aspire to belong to.
For example, an untouchable may belong to the Shudras due to the law of Karma and vice versa.