This form of current limiting is often referred to as Vbe current limiting because the threshold for current limiting is set by the Vbe voltage of a transistor. This is distinct from using a differential pair for current limiting which allows the threshold voltage to be arbitrary, more precise, and usually lower.
Ignoring the foldback part for the moment, when the voltage across emitter resistors R23 or R24 reaches the Vbe voltage of Q8 or Q9, about 0.6 volts, Q8 or Q9 turns on and removes base drive from Q12 or Q10 lowering the output current for that side.
The foldback part lowers the current limit further when the voltage across the output transistors is higher. In audio amplifiers, this is desirable because the reactive load of the speakers would otherwise drive the output transistors into secondary breakdown with higher currents at higher voltages. A purely resistive load would only result in high currents at low voltages across the transistors. Adding this allows less derating of the output transistors so they can be less expensive.