Hyperbole - a type of figurative language that refers to the deliberate exaggeration of something for dramatic effect.
Hyperbole allows a writer to draw attention to something by over exaggerating its importance.
The use of hyperbole can be both conscious and subconscious, although the effect of either is the same: the dramatic highlighting of something for the benefit of the reader.
Have you ever remarked after finishing lunch that This was the worst meal I've ever eaten, or come out of an English class declaring That was the best lesson I've ever had.
(Of course you have.)
These are examples of hyperbole, something we use all the time in everyday speech (often without thinking about it). In fact, in everyday conversation, it is rare for people to stick to what is actually the case, preferring to exaggerate wildly to get their points across.
In literature, hyperbole is everywhere, from Mark Twain writing in Old Times on the Mississippi: "I was quaking from head to foot, and could have hung my hat on my eyes, they stuck out so far", to Leo declaring in Titanic: "I'm king of the world!"
Hyperbole is not usually meant to be taken seriously, and it can be a fun and effective way to emphasise the importance of something in the eyes of the reader.
That said, hyperbole can also be used in serious or threatening situations to shock or motivate audiences. Many a world leader has used the line This is the gravest threat we have faced since ... in one of their speeches.