Plagiarism is when you use someone else's work in your assignment and pass it off as your own work. It can include not putting quotation marks around a phrase you have found in a book, using a whole essay written by someone else, using someone else's ideas, or copying something from the Internet and using it in your own work without giving the source.
Read the guide to citing literature references to ensure that you know how to write bliographies and quote sources correctly.
Bath University's Guide to Avoiding Plagiarism