Definition taken from Simple Wikipedia:
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.
In the Model United Nations, your resolutions will be made up of Clauses - i.e. parts of sentences. As such, the resolution should be able to read as one entire sentence. The clauses can be divided into two easily identifiable sections, the Preambulatory Clauses (not to be confused with Perambulatory) and the Operative Clauses. The definitions of Preambulatory Clauses and Operative Clauses will contain much more information about what you will need to put in to a good resolution.