This square is part of the ETH Zurich and is a popular hangout for many students before and afer lessons. It's a platform with a scenic view, perfect to: Watch fireworks, have a picnic, get some sun, read a while (for example about the architecture of Zurich), get a feeling for the size and structure of Zurich (especially the old town).
This is a spot with a beautiful view, and it can be reached within some minutes from the city. From Central, take the historic "Polybahn". When you arrive, exit to the right, and a few steps later, you arrive on Polyterrasse. This place warms up quickly thanks to its southwards orientation and is a nice place to relax on a sunny day.
Below Polyterrasse, there's a student's bar called BQM, a great place to grab a quick bite.