Relative clauses help to narrow the range of individuals to which a noun can refer, often to a single individual; like adjectives, they give additional information about the nouns they describe and for this reason some (especially older) grammars refer to them as adjective clauses.
‘the teacher who won the award…’
‘the teacher who lives in Hawaii…’
‘the teacher whom I met at the conference…’
‘the teacher I learned German from…’