Bright stars are close, and faint stars are farther away. This simple idea would work perfectly if all stars had the same intrinsic brightness. A 100 Watt light bulb is brighter than a 60 Watt light bulb. They both appear brighter if they are closer to you, and look dimmer if they are farther away, but the 100 Watt bulb is always emitting more light than the 60 Watt bulb. Or they might appear to be the same brightness.
Stars have wattages too, of course.
Boy, are we stuck. If we knew that a star that appeared bright was actually low-wattage, we would know that the star was close. Imagine studying a dim star and discovering that it was actually a high-wattage star. Well, we have used parallax to determine the distances of thousands of stars. If you know the distance to a star, getting its wattage is easy.