A Polyswitch is a form of Positive temperature coefficient resistor which acts as a self repairing fuse.
A polyswitch has a current rating . When the current flowing through the polyswitch, (which acts as a resistor) exceeds the current limit of, (for example) 100 mA, the polyswitch warms up above a threshold temperature and the resistance of the polyswitch suddenly increases from a few ohms to a few kiloohms.
When the polyswitch cools down again the resistance drops to its former level.