Citing your sources means that when you use the words or ideas of someone else in a paper or project, you must give proper credit. If you don't, it is plagiarism. Plagiarism is part of academic dishonesty and can have serious consequences.
The OWL at Purdue is a great online resource to consult about citations and formatting for your paper.
The UI Library catalog has a feature that will show you how to cite the article or book you are viewing in various citation formats.
Most of the library databases also have this feature. Look for a "cite" or similar link.