A clue, information, argument etc. that is or is intended to be misleading, diverting attention from the real answer or issue.

Examples

"I thought the issue with the car was the rumbling noise it made, but that was a red herring."

Etymology

One possible origin of the idiom was that red herring were used to train dogs to track scents. This was true[1], but the modern meaning of a false trail may have been popularised in a news story by English journalist William Cobbett, c. 1805, in which he claimed that as a boy he used a red herring (a cured and salted herring) to mislead hounds following a trail; the story served as an extended metaphor for the London press, which had earned Cobbett's ire by publishing false news accounts regarding Napoleon.[2] The OED has another possible earlier origin in the legacy of clergyman Jasper Mayne in 1672 when he misled a servant by leaving him "Somewhat that would make him Drink after his Death" in a large trunk. When the trunk was opened, the contents were found to be red herring. [3]

Sources

1. "Next, to draw on hounds to a sent, to a redde herring skinne there is nothing comparable.", Thomas Nashe: Lenten Stuffe (1599)
2. [2008, Michael Quinion, "The Lure of the Red Herring", World Wide Words.(http://www.worldwidewords.org/articles/herring.htm)
3. The Phrase Finder
4. Wiktionary