The Indefinite Leave to Remain UK is a type of visa of the U.K. In this type of visa, a person can get all the benefits of being a legal citizen of the U.K., and that too without any kind of restrictions.
A person can work over there, live over there, and also can study in the U.K. without any kind of restrictions. But in order to get this, I.L.R. visa, a person is required to live in the U.K. for at least 5 years already