If you are a beginner then you won't have to worry about relative clauses for a while.
Anyhow, a relative clause is a clause that is introduced by a relative pronoun (who, whom etc.)
There are three types defining, non-defining and connective. Although most people are only aware of the first two.
For example: "The man who told me this refused to give me his name."
'who told me this' is the relative clause here. In this case this is also called defining relative clause because if we leave out the relative clause from the sentence, it becomes too ambigous and will not be clea