Those are the steps that lead to entrance to the Acropolis proper. There is a short 5 to 10 minute hike to get up to those steps. The hike is also considered part of the Acropolis complex, because there are archaeological sites that date back to the building of the Acropolis. The steps are filled with people resting and enjoying the amazing view of Athens. When I was there, it was early August and the heat was overwhelming. So most people used the steps as a resting area before heading up to the main site.