When two identical repetitive patterns of lines, circles, or arrays of dots are overlapped with imperfect alignment, the light and dark lines that we call a moiré pattern appears. The moiré pattern is not a pattern in the screens themselves; rather, it is a pattern in the image formed in your eye. In some places, black lines on the front screen hide the clear lines on the rear screen, creating a dark area. Where the black lines on the front screen align with black lines on the rear, the neighboring clear areas show through, leaving a light region. The patterns formed by the regions of dark and light are moiré patterns.
In the case of the two sets of concentric circular lines, the dark lines are like the nodal lines of a two-source interference pattern. A typical two-source interference pattern is created when light passes through two slits. Along lines known as nodal lines, the peaks of the light waves from one slit and the valleys of the light waves from the other slit overlap and cancel each other. No light is detected along a nodal line.
In the black radiating lines of the moiré pattern, the black lines of one moiré pattern fill the transparent lines of the other. Note that as the patterns are moved apart, the dark nodal lines move together. This is the same thing that happens when light passes through two slits and the slits are moved farther apart.
Moiré patterns magnify differences between two repetitive patterns. If two patterns are exactly lined up, then no moiré pattern appears. The slightest misalignment of two patterns will create a large-scale, easily visible moiré pattern. As the misalignment increases, the lines of the moiré pattern will appear thinner and closer together.