What is foreshadowing in narratives?

Foreshadowing can take many forms. Either a seemingly insignificant item, fact, or comment becomes inherent to a plot twist later...or the character sees/hears/experiences something that tells the reader things are not what they seem. In some cases, it's nothing more than a feeling that something is not right, something's too easy...

The basic use of foreshadowing is to either cause a rise in tension or to skillfully weave in something that will become vital later. There's an old stage quote about not having a rifle mounted over the fireplace unless someone will be using it within three scenes. I don't specifically agree with that, since it may be nothing more than characterization (the character likes guns/is a hunter/is a military member or police officer...), but I will say that setting the rifle above the fireplace means it won't be seen as Deus ex machina (God in the machine) when someone needs to grab it down later. If the reader doesn't know it's there earlier, it feels contrived to suddenly have it be important.