You'd think that wasn't a question anyone would need to answer, especially in the field of journalism. But it does, because lots of people ask it. Or minimise it and shrug it off.
Some people seem to see plagiarism as simply cases you can look at and judge whether or not strings of text are verbatim. If people happen to know the author in question, or share their political views, sexuality, race or religion, it sometimes seems as if nothing short of dozens of examples of word-for-word lifted passages counts as plagiarism.
But that isn't what plagiarism is, and plagiarists rarely do that, anyway. Plagiarism is simply taking other people's words and ideas and passing them off as your own. Attributing your sources is very easy to do. Hyperlinking makes it even easier. People are taught this at school: if you want to take five sentences from, say, Wikipedia, that is shoddy enough, but you have to at least put the material in quotes or state clearly you got it from there. If you don't, you're rightly in trouble in school and you should be even more so as a professional journalist.
But why? Ideas are all floating in the wind in our post-modern world... blah blah.
No. Stop that hippy-dippy bullshit.
Ideas are people's work. They are researched, developed and crafted. Sources will also often need to be traced by others so they can figure out how stories have changed over time and weigh their reliability.
The reason plagiarism matters is because it is about the working methods of a writer, and if their ideas are stolen that is a problem. In the case of journalists, they rely heavily on readers' trust. I'm reading your interview with someone, but I wasn't in the room when it happened. So if you lie about what that person said, it is very hard for me to know. If you sensationalise, embellish or fabricate incidents out of whole cloth, as a reader I can't easily check. So I have to trust that you are being honest and scrupulous, and that when you report what people say in quotes that means they actually said those words, and that when you say something happened, you haven't embellished or invented.
So if I find you are so lazy and sloppy and uncaring about your work that you take material from elsewhere and act like you found it and wrote it yourself, you have hidden something very basic from me, your reader. And my trust in your ability to do all the other work of reporting evaporates.
So in all cases of plagiarism, it is not so much a question of counting: 'Oh, but that is simply a couple of examples. They made a mistake, big deal.' As soon as it is clear that this is not a coincidence, but a pattern in the writer's method, it doesn't matter if it is four examples or 804. If there is a pattern, that is plagiarism, and it is also very likely to be elsewhere in their work anyway. A newspaper or magazine or TV channel confronted with or finding several examples of plagiarism in a writer's work should immediately investigate all their work, and once they have done that fully and carefully explain what happened to their audience and either add disclaimers or remove the material as is appropriate.
Because if a writer is prepared to deceive their readers about how they came across their material - even if and perhaps especially if those facts are widely available - you can't really trust them to have told you the truth as best they can in their interviews, their experiences and in the rest of their work. That is why plagiarism matters.