- 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's written work for use, intentionally or unintentionally (usually the former), and not crediting them for said work, because of the intent to pass it off as their own or forgetting to give credit.
Because of the way that the internet works, plagiarism can be a problem, real or imagined, especially if someone has different handles at different fanfiction archives and uploads the same fic to both places, or they are actually plagiarized.
Plagiarism is bad.