Invention comes from the Latin word invenire and means "to find, to discover." Here it refers specifically to the way that you find and discover ideas and material for your papers.
In Aristotle's Rhetoric (one of the first great writing texts) we find that of the three skill areas most important to writers--invention, arrangement, and style--invention receives by far the greatest space: 60 of 82 pages in the W. Rhys Roberts translation, or, 73%. That figure should suggest the esteem in which invention was held in classical times. The emphasis has since changed: I surveyed 27 contemporary American "rhetorics" (textbooks used to teach writing) used for first-year English classes and found that the space allotted to invention averaged only 11%--a revealing statistic! What should matter to you, however, is that your professors will generally value invention closer to the ancient percentage than to the modern. At Guilford, original thinking and research are valued highly.
So how does one engage in college-level invention?