A clause is a unit of a sentence which contains a verb. Usually a sentence has a main verb, the verb without which everything else can’t make sense. The clause built around this verb is called the main clause. Sentences can have an arrangement (syntax) that is co-ordinated or sub-ordinated. This is the difference between ‘I walk and I talk and I laugh’ (co-ordination: lots of main clauses side by side) and ‘Because I was tired, and although I was nearly finished, I stopped (sub-ordination: two subordinate clauses, both reliant on the main clause and its verb, ‘I stopped). A typical sentence in Latin featuring sub-ordinate clauses would be something like this: ‘When he had finished speaking, the orator left the podium in order to greet the people.’ Here we have two subordinate clauses (‘when …’ and ‘in order to …’) and that main verb (‘left’).
In Latin the main verb will normally be in the indicative. Verbs in subordinate clauses will be in the subjunctive.