Multi-pass rendering effects use multiple renderings of the same frame, with slight camera movement between each rendering. The multiple passes simulate the blurring that film in a camera would register under certain conditions.
Multi-pass depth of field
Top: Focus is in the middle distance; near and far objects are blurred.
Bottom left: Focus on near objects, far objects are blurred.
Bottom right: Focus on far objects, near objects are blurred.
Above: Motion blur applied to wings of the flying dragon
Below: Multiple passes appear in successive refreshes of the Rendered Frame Window.