Definition:
Juxtaposition is a literary device wherein the author places a person, concept, place, idea or theme parallel to another. The purpose of juxtaposing two directly or indirectly related entities close together in literature is to highlight the contrast between the two and compare them. This literary device is usually used for etching out a character in detail, creating suspense or lending a rhetorical effect.
Example:
In Paradise Lost, Milton has used juxtaposition to draw a parallel between the two protagonists, Satan and God, who he discusses by placing their traits in comparison with one another to highlight their differences.
Example
“Should we weep when clowns put on their show”, is an example of juxtaposition.
One example to understanding a juxtaposition was when a movie played a happy tune while showing an extremely sad or upsetting scene.
In literature, you could use a similar idea such as describing one feeling while using a contrasting mood.