This crucial clue is one piece of important information that helps the main character finally solve the mystery. The crucial clue might be something that points directly to the perpetrator of the crime. Later in the story, the main character receives information about where all the suspects were at eight o'clock, remembers what Sam had said about receiving a call at that time, and knows that it couldn't possibly have happened. Your detective then realizes that Sam is the perpetrator. Red herrings are bits of information that are designed to mislead readers by making them suspect the wrong characters.
Red herrings are fun to include because they make mysteries harder to solve.
Maybe you want readers to suspect the main character's little brother, who has a real fondness for peanut butter and grape jelly sandwiches. Suppose your main character finds smeared jelly fingerprints in a suspicious place. Suspense is an important ingredient in a mystery story. Allow your characters to be scared. Your readers will identify with him or her, and they'll be scared, too.