A crowbar circuit provides reverse polarity protection by creating a short circuit if the polarity is reversed.
In normal operation, the resettable fuse F1 protects the attached circuit from overcurrent conditions, and the diode D1 is inactive. If the supply is connected in reverse, the diode D1 shorts power to ground, causing a large amount of current to flow, and tripping the resettable fuse.
A more sophisticated crowbar design is possible using an SCR and a Zener diode that also protects against overvoltage in a similar fashion.