What it shows:
The criterion for the resolution of two sources is that the central maximum of the single slit interference pattern of one source falls on the first minimum of the pattern of the second source.
How it works:
Each laser beam passing through the slit will form a diffraction pattern on the screen. With the aperture closed down, the pattern will be spread out and the central maxima of both sources will overlap giving a blurry image. Opening up the aperture and the diffraction patterns will get narrower, until the point...
Read more about Rayleigh's Criterion