While it is widely acknowledged that string theory provides a framework for addressing quantum gravitational (and even quantum field theoretic) questions, the precise nature of its organizational principle remains obscure. Nearly three decades ago, when the question “What is string theory?” was asked, the perturbative string worldsheet viewpoint played the starring role. Since then, the discovery of D-branes, string dualities, and the holographic AdS/CFT correspondence has had spacetime dynamics taking center-stage. Much insight has been gleaned from these developments. We now have, for example, a much broader class of string backgrounds with better control over compactifications; strong constraints on the low energy effective field theories; a plethora of gauge-string dualities as well as applications of holographic techniques to a wide class of physical problems; and new angles on semiclassical gravitational physics from a fruitful interplay with quantum information in holography. Together with an influx of new tools and techniques, we have a very rich set of perspectives that illuminate the subject we continue to call string theory. While all these developments are interesting in and of themselves, it is an opportune moment to attempt to synthesize these, often disparate, strands and arrive at a deeper answer to the question in the title.

We anticipate the program having six broad themes. 

1. The nature of spacetime and role of quantum information at the string scale. 
2. Developments in string field theory and dualities.
3. Deriving dualities and new approaches to QFT.
4. Holography beyond AdS. 
5. Stringy insights into effective field theories. 
6. Progress in string compactifications.