The coloured fringing you see at the edges of a photograph taken with a cheap lens. Glass bends different wavelengths by slightly different amounts, so red, green and blue never quite land in the same place.
The effect reproduces that by pulling the red channel one way and the blue the other, leaving green where it is. Crucially the amount grows with distance from the centre of the frame, which is how real lenses fail: dead centre everything lines up, and the further off axis you go the worse the separation gets.
What makes this more than a channel shift is what happens next. Instead of returning the shifted channels directly, the effect subtracts the unshifted image to leave only the difference, which is the coloured fringing on its own, and then adds the original back scaled by passthru. That lets you push the fringing hard while keeping the underlying picture at whatever strength you like.