In the conjuction specifically in subordinate conjunction we will find clauses which will be complete or incomplete idea. When a clause is already contain subject and verb that can be considered as complete idea or independent clause, but the complete idea can be incomplete idea or dependent clause when they're added with subordinate conjunction.
example:
He is eating when his sister read a book.
The word 'he is eating' is a complete idea/ independent clause because we don't need any explanation of his action.
While the word 'when his sister read a book' is dependent clause because it need more explanation about what happen when she is reading a book. This clause is actually independent clause, but whenever we add subordinate conjuntion in the independent clause it will be dependent clause that need another clause to rely on to make a compete sentence.