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 clear which man we are talking about.
Now it has probably occured to you that from a non-defining relative clause you can leave out the relative clause without compromising the meaning of the sentence. The only thing you have to pay attention is to enclose the non-defining relative clause between commas.
Connective clauses don't describe their nouns, just continue the story.
i.e. I told Steve, who said it wasn't his business.
Use the RC after the subject of the main verb.
Hope that helped