- ⧽ So that no room is here for writers left,
⧽ But to detect their ignorance or theft. - —Andrew Marvell, "On Mr. Milton's Paradise Lost"
Plagiarism is the act of stealing another person's written work with the intent to pass it off as your own. Extensively quoting or otherwise borrowing from someone else's work without acknowledging your source also counts, even forgetting to give credit was an accident. Plagiarism is bad.
Because of the way the Internet works, plagiarism can be a problem, real or imagined. It's all too easy to post someone else's work in a new place with a new name and pretend it's yours, or to do the same thing yourself and pretend you've been a target. Meanwhile, writers with different handles at different fanfiction archives have been accused of plagiarizing their own work.
A common problem encountered in badfics is canon regurgitation, in which all or most of the plot of the canonical source material is rehashed or copied in a fic, sometimes word-for-word, with only negligible original content (often, the addition of a single Mary Sue). This is also a form of plagiarism and, since most canon works subject to this are professionally published, copyright infringement to boot.