Plagiarism is the act of using someone else’s work without their permission, and passing it off as your own.
It’s fine if you refer to other people’s work in your own: if you’re reviewing a book or writing an essay, for example, you’re allowed to quote from other works so long as you acknowledge your sources fully, the essay or review consists mostly of your work and the quotes are only a small part of it, and the quote you use is short enough to be considered fair use.
It is not acceptable for a writer to take another writer’s work and pass it off as their own. It’s no defence if you put the piece through a cursory rewrite; nor is being unaware of the plagiarism laws. That’s plagiarism, and publishers and courts take a very dim view of it.