Plagiarism is when you copy someone else's work and pass it off as your own. So, for example, if you did a copy/paste of an answer on enotes and turned it in to a teacher, that would be plagiarism. Even if you changed it slightly, if what you turn in is a close imitation of the other person's words, you've crossed the line.
Plagiarism is a really serious offense. Students have been expelled from schools and professionals have lost their jobs as a result of this error.
If you plagiarize something you might ALSO be guilty of copyright infringement, although they are separate issues. Copyright protects the rights of the original author. Plagiarism is focused on the undeserved credit you get from supposedly having written the piece.