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.