In simple terms, plagiarism is passing off someone else's work as your own. For example:
Sometimes students plagiarize intentionally. They run out of time to complete an assignment or they're worried about their ability to complete an assignment successfully or they get lazy, and they copy information from a source.
Obviously, intentional plagiarism is a serious academic offense, and students who commit this type of offense face serious academic consequences.
Sometimes students plagiarize unintentionally. They don't have a great deal of practice at paraphrasing and documenting correctly, and they make errors.
When unintentional plagiarism occurs, it can represent a learning opportunity for students -- a time to seek help from the instructor and additional information about documentation styles and how to use them appropriately.
Please note, though, that even unintentional plagiarism can result in serious academic consequences - depending on the specific context within which the plagiarism occurs.