Overloading of electric circuit means that the current circulating in the circuit becomes more than the capacity of components in the circuit to withstand the current. All components in the circuits have some resistance passage of electricity through this resistance produces heat which is directly proportional to the square of current flowing. The components are designed to withstand only that much of heat as is generated by maximum designed current. When the current is more than this level, or in other words when the circuit is overloaded the components get overheated, leading to their damage.
Frequently electric circuits in homes, factories, and other electrical installations incorporate fuses, which are essentially circuit component which protect other components in the circuit by quickly melting or burning out when the circuit is overloaded, resulting in breaking of the circuit. In common language this is called blowing of fuse.