Perhaps the clearest analogy that can be made to describe the role of the rough endoplasmic reticulum is that of a factory assembly line. Along its surface, which Dictionary.com describes as "rough" owing to the ribosomes arrayed along its membrane, proteins are synthesized and transported for use in the cell's chemistry.
The rough endoplasmic reticulum is a long, convoluted structure inside the cell that is folded into a series of sacs along its course. It connects the nucleus of the cell with its plasma membrane and is often intertwined with the smooth endoplasmic reticulum, which has a subtly different function and is folded into a more tubular shape than the rough endoplasmic reticulum. Proteins and, in leukocytes, antibodies are assembled along the length of the rough endoplasmic reticulum and conveyed to either end for use in the cell's biology, according to About.com.
From the description About.com gives, it is easy to imagine the components of various proteins, such as insulin, being conveyed along its surface and assembled as if they were cars in a factory. Once the proteins reach the end of their journey, they're ready for use in maintaining the body's chemical balance and are released into the cytoplasm.