The power dissipated by a resistor is the voltage across the resistor multiplied by the current through the resistor.
If the average power dissipated is more than the resistor can safely dissipate, the resistor may depart from its nominal resistance, and may be damaged by overheating. Excessive power dissipation may raise the temperature of the resistor to a point where it burns out, which could cause a fire in adjacent components and materials.