Dedicated to Dariusz
A herring is a fish, but a red one?
It's a strange idiom indeed to describe some kind of false clue or trail.
Agatha Christie's crime novels are full of red herrings.
Now, I really recommend that you view the next video clip with a really excellent explanation of this idiom from its literal origin through to how it might be applied in an argument.
Who would have thought fish could be so much fun!!