A red herring is a metaphor for a diversion or distraction from an original objective. An example can be found in academic examinations, particularly in mathematics and physical sciences. In some questions, information may be provided which is not necessary to solve the given problem. The presence of extraneous data often causes those taking the exam to spend too much time on the question, reducing the time given to other problems and potentially lowering the resulting score.
Red herrings are frequently used in literature and cinema mysteries, where a character is presented to make the reader/viewer believe he/she is the obvious perpetrator, when in reality it is someone far less suspect.

The phrase may relate to saving a hunted fox by dragging a smoked herring across its trail. This act would create a diversion through the strong smell of kippers. The Oxford English Dictionary records its first written use occurring in 1686 in this context: "To draw a red herring across the track". Michael Quinion says it is unlikely that any such act ever occurred.

Medical: An unusual clinical, radiologic or pathologic finding that should be ignored in the context of a Patient's disease. See Sutton's law.

The definitions seem to relate the color red to the herring's having been smoked. I had always thought that as the fish spoiled, it turned red; so a red herring was one that was really sinking rotten. Therefore it was perfect to use to put a tracking dog off the scent.

I certainly would not waste a good, expensive kipper by dragging it around on the ground. But using a stinking rotten fish that way -- no problem. One could even have their small children drag one along; it would be more effective at finding them than a GPS transmitter, and cheaper.