PTC thermistors can be used as current-limiting devices for circuit protection, as replacements for fuses.
Current through the device causes a small amount of resistive heating. If the current is large enough to generate more heat than the device can lose to its surroundings, the device heats up, causing its resistance to increase, and therefore causing even more heating.
This creates a self-reinforcing effect that drives the resistance upwards, reducing the current and voltage available to the device.
Mounted in series to the speaker, they are well suited to protect speakers against overload.