The proletariat is a term used by communists and socialists for the working class. It describes the people in society that sell their labor/work in exchange for a wage or salary. Someone who is part of the proletariat class is called a proletarian.
The political way of thinking of communism states that the proletariat should have a revolution and take control of the means of production away from the bourgeoisie.
The proletariat is the laboring bulk of society that does most of the useful, necessary work, and pays most of the taxes. It is also perceived by capitalists as being necessary for those reasons: labor and taxes, plus being available to exploit to produce profits.