Plagiarism means “to steal and pass off (the ideas or words of another) as one's own” or to “use (another's production) without crediting the source” (Mirriam-Webster.com). 

Certain forms of plagiarism are outright cheating, as when a student buys a paper from a paper mill or copies all or part of another student's paper.   However, other forms of plagiarism may involve a misunderstanding of the purpose of a research paper and of citing sources.  Students should think of their research papers as part of a larger conversation. You are coming into the conversation after it has begun, so you need to bring yourself up to speed on what has been said by other scholars.  That's the research and that’s what you need to document with appropriate citations (see Tips for Avoiding Plagiarism).  When the professor is able to hear your voice in a well-researched and well-documented paper, that’s how he/she knows that the learning has been achieved.