The original position in one in which, for the purposes of Rawls's thought experiment, everyone is kept behind a veil of ignorance. This means that we are unaware of our gender, race, social position, or any other factor which might determine our place in society. It is in this original position that the all-important choice as to what kind of society we wish to see established should be made.
Rawls is convinced that, if we occupied the original position, we would naturally opt for a fair and just society. This is because, on Rawls's account, the original position provides us with an impartial point of view. As part of this thought experiment Rawls invites us to imagine that we are all free and equal people jointly committed to principles of social and political justice. Once we've done this, then Rawls believes that we will make what he regards as an appropriately rational choice, choosing to live in a society in which freedom and equality of opportunity are maximized to the greatest possible extent.