I used to hate the word 'juxtaposition'. It always sounded like a highfalutin way of saying 'positioning'. Over time, though, I came to understand that there is a difference.
Juxtaposition is the positioning of things relative to each other. In visual arts, it can easily be seen in comparison and contrast. A classic example would be showing two pictures - one of a starving child and another of an obese one. The juxtaposition shows the vast extremes the human form can take. It can show the vast extremes in socioeconomic levels as well.
When we tell stories, we are also using juxtaposition. When tragedy happens, the world becomes tragic. It always rains at a funeral. Sad music plays when you randomly turn on the radio after a breakup. Once you get fired from your job, your car also breaks down and a bird craps on you.
The neat thing is that, no matter what you do, you are creating a juxtaposition. The standard technique is to build a scene of similar themes, but if you make contrasting themes (or try to avoid it by being realistic), you are still creating an image.
When a man comes home to find out that his wife left him and took their son with her, it would be really awful to turn on the TV and see a commercial for the local day care center, filled with smiling faces and excited voiceovers. However, the juxtaposition could make for a very powerful scene, emphasizing the significance of the loss.
More likely, though, in the above situation, it would be a car commercial. It would have nothing to do with the man's situation and the mood would not reflect the tone you have created. The juxtaposition of the commercial with the scene would only show that the man's life is insignificant and that the world will carry on, no matter what happens to him.
Writing, no matter how realistic it appears, is not real. It is an artistic expression with the intent of either creating a particular thought, or to compel people to create their own thoughts. You, as a writer, are charged with making that happen. Juxtaposition is one of your countless tools.