A caste system is a class structure that is determined by birth. Loosely, it means that in some societies, the opportunities you have access to depend on the family you happened to be born into.
The phrase caste system has been around since the 1840s, but we’ve been using caste since the 1500s. You might’ve heard about the caste system in India, but there are caste systems all over the world, where people are born into a position and social mobility is difficult to achieve. The United States prides itself on not having a caste system because people can "pull themselves up by their bootstraps" — although some people that there is an implicit caste system in American society.