If something is effectual, it actually produces the result you wanted. It does not only look useful, or sound strong; it works. You can say an effectual plan, an effectual warning, an effectual law. It is a formal, slightly old-fashioned word. In everyday talk, people almost always say effective instead, and that is the safer choice in school writing too. Though actually, you will meet the opposite more often than this word itself: ineffectual — a person or an action that tries, but does not really change anything, like a weak protest or a leader nobody follows. In older or legal English, effectual can also shade toward “valid; enough to count,” as in an effectual agreement, but the core idea is still the same: it does the job.