Colloquialism:
The use of slang or in-formalities in speech or writing, including local or regional dialects and usage. 

Example:
"If I must be sold, or all the people on the place, and everything go to rack, why, let me be sold. I s'pose I can b'ar it as well as any on 'em."
-Uncle Tom's Cabin, By Harriet Beecher Stowe

Explanation: 
Uncle Tom's Cabin was classified as anti-slavery literature. The language/colloquialisms in the novel shows the time and place that it was set in. The language reflects how the African Americans spoke during this time, showing they were ignorant and not treated like humans. Which is one of the main purposes of the novel.  The main purpose of these colloquialisms give the reader a glimpse of what people were like, and it made the reader want to do more to change this.