plz can anybody tell me what is non-defining relative clause

Relative clauses are subordinate clauses that add information, define or identify the noun. They are also known as adjective clauses. Clauses starting with the relative pronouns 'who', 'that', 'which', 'whose', 'where', 'when' are relative clauses. 

Here are some examples:

• He is the man who was gave the speech yesterday.
• This is the school where I studied.
• She was searching for the book which was in her hand.
• She offered a proposal which Nick did not accept.