A clause is a part of a sentence. Each clause has only one main verb. I love you is a sentence which has only one clause. I love you and I will always love you is a sentence which has two clauses. The two clauses are I love you and I will always love you. These clauses are joined together by the word and (a conjunction).
Two clauses can be joined with a pronoun. For example: I live in London, which is in England. Here, I live in London is the first clause, and which is in England is the second clause. The word which is a pronoun which takes the place of London. It joins the two clauses.
A sentence can contain many clauses. But sentences with fewer clauses are easier to understand.
A subordinating clause is one which is dependent on the main clause (the primary clause). The main clause makes sense on its own. However, the subordinating clause does not. E.g. I love you makes perfect sense left on its own. However, and always will, does not.