In an optical system, the AA filter basically blurs the image slightly by passing the light through two layers of a birefringent material (lithium niobate). A birefringent material separates incoming light into two rays, essentially polarizing it. The two layers are oriented orthogonally to one another. There's a 1/4-wave plate in between the two layers to circularly polarize the light after it passes through the first one (else it would be blocked by the 90° polarizers). The thickness of the layers determines the degree of separation of the light rays, and that thickness is varied according to the pixel density of the sensor, to match the Nyquist frequency of that sensor.