Power flow is made up of two parts, voltage and current. Electricity flowing through wires is just like water flowing through a pipe. The pipe has water and pressure that pushes it through. In electrical terms, current is the water, and voltage is the pressure. If either one increases too much, it can "flood" your electrical system and cause damage.
Your home's current is either 100 amps or 200 amps, and the correct voltage is 240v. So power surges occur when there is an increase in the flow of voltage and current into your home beyond these amounts. Power surges can be caused by lightning, transformer malfunctions, shorts in the electrical infrastructure, or when the system comes back on after an outage. These problems are all out of your control.
When power surges occur, the excess voltage can fry your appliances, and excess current appears in the form of heat, which can melt your wires and cords. This is why installing a power surge protector is an affordable and important home protection option.