When an electric heater is switched ON in a parallel circuit, it draws a large current due to its low resistance. This increases the total current drawn from the source, which causes a greater voltage drop across the internal resistance of the source. As a result, the terminal voltage (potential difference available to the circuit) decreases. Since the bulb now receives a lower voltage, the current through it decreases, and it glows dimmer (as power P = V²/R decreases).
Examiners expect two key points: (1) the heater draws large current, increasing drop across internal resistance, and (2) this reduces the voltage across the bulb, reducing its power/brightness. Mention of internal resistance is important — without it, voltage across parallel branches should stay constant, so the drop across internal resistance is the crux of the answer.