This is a great question. Here is a brief historical sketch. The word, proletariat comes from the Latin word, proles, which means offspring. According to Roman society, people were broken into classes by wealth. Senator were at the top, then came the equestrians and at the lowest rung of the social ladder were the proletarians. They were those who could only give their offspring (proles) to the state. So, they held little or no property and their benefit to the state was their labor and their children.
Perhaps, this idea of the proletariat is more familiar with us because of Karl Marx. In his theory, the proletarians are the working class in a capitalistic society. They are the ones who do not have any ownership in the means of production. Their only means of production is to sell their labor for a salary, which is often very low. In short, they are the massive working class that the wealthy in a capitalistic framework exploit.