He is one of the greatest writers if not the greatest writer
What does it mean? I am confused and cannot understand it. Please help me
Try reading it with this emphasis:
He is one of the greatest writers, if not the greatest writer
The distinction is in writer vs writers. The word "greatest" is a superlative. Other examples of superlatives are best, fastest. If you were to rank all writers according to greatness, the greatest writer would be one single writer at the top of the list. The greatest writers would be an unspecified number of writers at the top of the list. The subject of the sentence is (in the author's opinion) amongst the top ranking writers and may even be at the top of the list.
What is unclear about this sentence is how "greatness" is measured. Different people will have different opinions on what constitutes a "great" writer, so it must be taken as an opinion rather than a fact.