What is pass rendering in 3D., Any one knows its

Render passes or render elements, are the raw elements generted by rendering engine. It is composited together for you to the final output render.some of these elements are Lighting, Shadow, Reflections,specular and global illumination just to mention a few.

Render passes are renders of your scene in different attributes separately. Like you can render the reflection only or an specific material only. This can help a lot in post rendering phase.