As a socioeconomic class, the proletariat has been around for awhile. The word itself comes from Latin and referred to a class of Romans who didn’t own property. While the nature of this class has morphed through time (whether farmers working on someone else’s land, workers in someone else’s factory, etc.), the primary element of the proletariat is unchanged: its members’ only real material value is in their labor. Generally speaking, they own little or no property and work for someone else.
The idea of the proletariat as a “genuinely revolutionary class” comes directly from Karl Marx’s Communist Manifesto, published in 1848. Marx (and his co-author, Friedrich Engels) argued that there were two essential, competing classes in society: the bourgeoisie (who owned the means of production, particularly land) and the proletariat. Any other classes (i.e., the various forms of the middle class) “decay and finally disappear in the face of Modern Industry; the proletariat is its special and essential product.” Marx considered the middle classes conservative, as they fought to keep their middle-class status and avoid becoming part of the proletariat (a shift Marx considers inevitable in the industrial age).
The proletariat, however, are constantly exploited by the bourgeoisie for their labor but never truly reap the fruits of that labor, as the profit goes to those who own the means of production: the bourgeoisie. There is a real and significant lack of social and economic mobility for the proletariat; generally speaking, proletarians are unable to rise into a higher social or economic class. The proletariat is also the largest class of the population.
Because of these facts, Marx argued, the proletariat are the prime option for revolution. It is crucial for the proletariat to rise up and seize the means of production (the biggest of which was land but which could also be basic property, manufacturing tools, etc.). Only then could they live a life that was not full of exploitation and “pauperism” (poverty). Marx thought that this revolution was both necessary and inevitable, due to the rise in capitalism, industrialization, and automation—similar ideas that persist today with the rise of technology displacing jobs.