Today is the second and last day of the YRS. The first talks were in areas of mathematical physics that I am not informed enough to comment on. One was on some sort of random walks, the second on entanglement and the third was too mathematical. You may see the program here.
The most expected talk was given by Witten and was about the Langlands program. He started giving an elementary introduction to electric-magnetic duality, magnetic monoples, Dirac strings and the quantization condition to arrive at the Montonen-Olive duality. He then explained the goal of the Langlands program, that is, to study number theory using geometry, and ended mentioning his work, that the Langlands program can be interpreted in terms of physical ingredients. Since the audience was very heterogeneous it seems he did it well.