What is the function of a fusible plug in a Boiler ?

The Fusible Plug is a safety device in a boiler.  When the temperature of the boiler reaches abnormal levels, the plug fuses (melts) and provides an exit for the steam.  The jet of steam which exits the boiler will alert the operator to the high temperature.

A typical scenario is when the water level in the boiler falls below the safe limit.  This can cause the boiler to overheat and ultimately fail due to the metal walls softening. 

The plug is placed below the water level.  If the water level falls below the plug, the boiler starts to heat and the plug operates, releasing steam.