Somebody keeps editing my posts on Huggett. The good news is that the edits are truth-preserving.
Huggett now is plowing through his simple classical example, the solutions for the string’s behaviour and stress-energy. One thing that pops out are the Virasoro constraints, which correspond to the conformal symmetry of the string on the worldsheet, which Huggett mentioned earlier. Using these constraints one can show that the mass is a function of (or “comes from”) the vibration modes of the string.
Beyond Spacetime 
The two main points up to now are: (a) that the sigma action (equivalent to the N-G action mentioned below) involves a 1+1-D metric, h, over the string worldsheet – but its absence from the N-G action indicates that it is introduced as a trick, so that there is conformal symmetry wrt h – worldsheet distances have no physical significance. (Unlike target space distances – only Lorentz symmetry there.) And (b) the action corresponds to a basic wave equation, so that everything you’d expect goes through: the solution is
, and when you quantize the modes become raising and lowering operators. In short, the math, and perhaps the ontology, it just that of a QFT on the worldsheet.