Light is energy, which is measured in watts, but what matters for us is the amount energy that falls on the specific area of our eye. Thus, we measure the energy that falls on a given area as a measure of the light intensity. Thus we talk of brightness, or luminous intensity. The area of our eye is fixed. If we stand close to a light, it seems bright. If we move a long distance away, it is much dimmer, or we may not see it at all. As the light spreads out, so the energy falling on our eye gets smaller.
So we can measure luminous intensity in terms of watts per square metre, say. Now, in physics, it has been shown that luminous intensity falls off with the inverse square of the distance between source and observer. What does that mean? Suppose we have a square, white screen that is exactly one metre along each side. If we hold the screen 1 metre away from a light source, let's say the energy falling on that square metre is 400 watts. If we now move the screen to a distance of 2 metres, the light intensity falling on the screen is now 400/(2 x 2) = 100 watts. Increasing the distance to 4 metres results in a decrease of the light to 400(/4 x 4) = 25 watts. And so on...