In the first Harry Potter book, Hermione uses a spell to open a door. This same spell must be used later when the famous trio is searching for the Sorcerer's Stone. This is an example of Chekhov's gun. If you add one to your book, you'd better darn well be ready to fire that gun. At least, that's what Anton says.
No, I'm not just talking gibberish. I'm talking about a quote from Anton Chekhov, considered by many literary experts to be one of the greatest short story writers in the world. Here's what he says:
"Remove everything that has no relevance to the story. If you say in the first chapter that there is a rifle hanging on the wall, in the second or third chapter it absolutely must go off. If it's not going to be fired, it shouldn't be hanging there."
And I absolutely agree with that. You don't want to weigh your readers down with a bunch of extra details. If there's a laptop sitting on the desk by the wall, then I want to see the character at that desk using that laptop at some point. Otherwise, why did you tell me about it?
A seemingly random detail, usually inserted early into a story, which later comes into play is Chekhov's gun. It's a form of foreshadowing, and like any literary technique it's only powerful if it's revealed the right way. You need to draw an appropriate about of attention to the "gun" in question, but try not to build your readers a neon sign that it's going to be significant.
The important thing is, make sure Chekhov's gun goes off. Make sure that every detail with a start point also has an endpoint. Otherwise, Chekhov's gun becomes a MacGuffin...and then you're dealing with a whole new literary device.