Plagiarism means many things, but in simple terms it means putting forth someone else's work as your own. As a Cornell student, you agreed when you enrolled to be subject to the Code of Academic Integrity, which forbids plagiarism in any form.
Many students are confused or nervous about how to avoid plagiarism or are afraid of accidentally committing it. However a few simple rules can help you to avoid it. Take a look at these links and this video below (courtesy of Bainbridge State College) for information about plagiarism.