The Flash on your camera is daylight balanced so when used correctly a flash can add great additional light to a photo.
A flash can add sparkle in the eyes of your portrait subject & can freeze action in mid flight - so have fun getting to know your flash.
Effective Flash Distance
The goal of adjusting your flash power is to have your subject receive exactly the nicest amount of light
Every camera flash has a maximum effective range - check this in the specifications.
Dedicated Flash
This is a flash that can communicate with your camera via TTL so the flash knows what settings the camera has and can be compensated via the on camera menu.
Automatic Flash
Some flashes can be set to Auto even though they cannot communicate with the camera.
You then adjust the aperture on the camera to allow to match the distance to subject.
With the flash shown below if you were running at 100 iso and your subject was 8 metres away you would set the aperture to 6.6 (or thereabouts f5.6) - run full manual and set your shutter to between 60th and 250th
Off Camera Flash
Many modern Flashes can be remotely activated as a Slave Flash with Wireless or Flash signal from a On Camera Flash or Dedicated Master Flash - You can also add a Wireless Flash Trigger to your Hotshoe and a Slave Flash Receiver to your Flash Hotshoe.
Flash Compensation
To create the most stunning photos with the use of Flash, you need to be able to compensate the power - usually less so that the Flash is just enough to highlight the subject.
How to compensate the Built in Flash on a Canon EOS DSLR
How to compensate the Built in Flash on a Nikon DSLR
How to compensate a External Flash on a Canon EOS DSLR
How to compensate a External Flash on a Nikon EOS DSLR
Have fun experimenting with compensating your flash power and remember the surroundings make a huge difference
eg. You will need much less flash in a small white room than you would in an open field with nothing to bounce the flash off.