Plagiarism is a form of cheating. It essentially boils down to inappropriate use of information in your work or failing to provide an complete citation for material you have used, leading to the impression that it entirely your own creation.
It is often a result of poor referencing skills or not realising what you are doing is actually wrong.
Have a look at the links below, which explain what plagiarism is and provide tips on how to avoid it.
This video from Bolton University explains what plagiarism is and how you can avoid it.