When a scene combines ambient light and light from a flash this is a way to control the balance between the two.
The difference between the ambient light and the flash is this: The flash is a very short burst of light much shorter than the time the shutter is open (assuming you are shooting at or below your camera's flash sync speed). The ambient light, on the other hand, is constant over the entire time the shutter is open.
The changes you make to aperture and sensitivity (ISO) will affect both the objects illuminated by the ambient light and the objects illuminated by the flash. But any changes you make to the shutter speed will only affect the objects illuminated by the ambient light. To adjust the exposure of the object or person illuminated by the flash, you adjust the power level of the flash instead of adjusting the shutter speed.
To understand how the shutter speed is irrelevant to exposure in terms of the flash, imagine that you are in a totally dark room. If the flash is illuminated for 1/1000th of a second, it doesn't matter if the shutter was open for 1/200 second or for 200 seconds - the same amount of light will be captured in both cases: the light that was present for only 1/1000 second. (With digital, there might be additional noise generated by the heat produced by the sensor staying energized for that length of time. With film there would be absolutely no difference.)
Now, back outside to our portrait subject. If you want to shoot manually, first meter to determine the proper exposure of the background and set your camera's ISO, Aperture, and Shutter speed to match that exposure value. Then meter to determine the proper exposure of the subject, compute the number of stops difference between the two and add enough flash to make up the difference. What power level you will need to use for the flash will depend upon several variables: The flash's Guide Number that indicates how powerful it is, the distance from the flash to the subject, and the aperture and ISO you selected when metering the background.