Considered in relation or in proportion to something else. The cost of living in the city is high, but it is relative to the average income.
Dependent upon or interconnected with something else. The success of the project is relative to the amount of effort put in by the team.
Having significance or importance only in comparison to something else. The importance of the discovery is relative, as it depends on the context in which it is used.
Grammar: used in a relative clause or relative pronoun phrase; referring back to a noun or pronoun. 'That' is a relative pronoun in 'the book that you lent me'.