Pick out the criminal in this lineup:
Which one do you think is the guilty party? Maybe this long-haired guy in the center? How about this Amish-looking fellow in the end? Maybe the guy with the short hair is trying to fool everyone, and he's the real perpetrator. From where I'm sitting, they all look like wrongdoers to me.
But I'm actually the one who's wrong.
This is a photo of the Beatles, not potential criminals. But when you look at the picture the right way, they look like they're up to no good. This is how a red herring works.
It's not a fish when it's used in fiction. In fiction, a red herring is a person who looks absolutely guilty. You know they're the one who committed the murder, or stole the painting, or cheated on so-and-so, or whatever. But what you don't know is that you're seeing the character through a distorted lens, and a clever author is actually fooling you into thinking that person is guilty.
They're totally innocent when they're a red herring - that's what it's designed to do. You're busy suspecting this person and the real killer (or whatever) is hiding right there in plain sight on the page, probably charming you. Probably wooing you. And perhaps making you their next victim (not literally...just literarily).
Red herrings are, most often, characters. Sometimes they're objects, however, such as an item that appears to be the murder weapon but something else is actually the murder weapon. Red herrings are most commonly used in mysteries in order to lead you away from the real killer, but they can be found in many different types of fiction.
A really good red herring won't start to stink until the end of the book, when the real truth is revealed. In the best-case scenario, the distorted lens is swept away and the red herring can now be viewed clearly. That sinister maid with the ulterior motive becomes a hard-working single mother. The professor who knows perhaps a little too much is simply a well-read, socially awkward man with a secret crush on one of the other characters. You get the idea. The point is, readers shouldn't suspect that something is fishy...until you want them to.
Start sprinkling red herrings into your books, and see if you can misdirect and divert your readers long enough to keep them guessing until the very last page.