I must admit that I do not like applying the terms “defining” and “non-defining” to relative clauses because the so-called “defining” clauses seldom or ever define a noun. I prefer the terms “descriptive” and “restrictive.”
A descriptive clause adds information that is not essential to the main sense of the sentence. It could be left out.
Ram is a very good boy
is a complete thought.
whom I called yesterday
is a separate thought. It merely describes an incidental fact about Ram that is not relevant to whether he is a good boy.
A restrictive clause adds information that is essential to the main thought
The policemen who arrived at the accident were efficient in their action and also very polite.
The clause starting with “who” is essential to the intended meaning. No statement about policemen in general is intended. It is a statement restricted in meaning to specific policemen at a specific time and place.
In some cases, it is permissible to omit “that” or “which” from the beginning of a restrictive clause. That is not true of clauses that are merely descriptive.