Causal clauses are subordinate clauses that express the reason or cause behind an action described in the main clause. In Latin, these clauses often begin with conjunctions like 'quia' (because) or 'quod' (that), and they provide insight into the motivations or justifications for the action taking place. Understanding causal clauses is essential when analyzing mood usage, as they typically employ the subjunctive mood to indicate hypothetical or contingent situations.