You would use external flash in order to create a balance in lighting, when you want your subject as bright as the background or when you want everything in the image to be lit equally.
If your using flash, you do not need to meter light.
Flash synchronization is when the flash goes off at the immediate time a picture is taken.
ETTL means when referring to a flash that it works through the camera lens, adjusting to its settings.
If you shoot faster than the camera sync speed the camera shakes, and the image becomes blurry.
Photographers might bounce a flash or use a diffuser to make the lighting softer.
A slave is something that sets off a second flash.
If an image comes out either to light or to dark, you should mess with the aperture.