Working class is a term used to describe people in society who have jobs which are not academic or highly paid. They're usually jobs which are physical, especially when they are paid an hourly wage.
Different societies will have different ideas about what "working class" is. People started talking about the working class after industrialization in the early 19th century. Karl Marx wrote about the working class.
Working class contrasts with middle class and upper class, which are above working class. The underclass is below the working class.