Proletariat is an old term for the working class. It was commonly used by Marxists and other people who believed that capitalism had created a class of workers who were exploited by company owners. They called those owners "the bourgeoisie."
When you use the word proletariat now, your audience might assume that you've arrived in a time machine from the 19th century and you're still angry about the exploitation of workers. So you probably would use the term self-consciously or humorously, as in “The boss just sent around a memo saying no one’s allowed to leave until we’ve finished compiling the report. I guess the proletariat had better forget about making it home in time to watch the kick-off.”