Of course, most students will say that they already know what plagiarism means. But do they really understand the difference between legitimately paraphrasing content and plagiarising it? Or how to cite a source correctly?  Spending some time NOW may go a long way towards preventing future problems and reducing the likelihood of “mistakes”.
Without thorough understanding, it may be difficult for students to appreciate the seriousness of plagiarism. Their response may be: “How can copying some words actually hurt anyone?” But the reality is that plagiarism is an act of fraud that involves both stealing (another’s intellectual property) and lying (implying that the work is one’s own).  This undermines the principles of trust and respect that make education possible. When they plagiarise, students harm more than just their lecturers and the person from whom they steal. They also harm themselves, because they fail to acquire the research, analytic and writing skills that they would have learned by doing the assignment honestly. Finally, it is also unfair towards those classmates who have legitimately earned their grades (and degrees), and with whom you will be competing for jobs.