Guide Numbers and sync speed in flash.
These are an indicator of power. High numbers = more power. More power = more reach.
With flash exposure is calculated based on flash to subject distance not camera to subject distance.
If the guide number is known ( usually stated in Metres at ASA / ISO 100 ) then aperture can be manually calculated by dividing the guide number by the distance in metres. You can therefore, as long as you know the guide number of your flash work out its reach within the limits of your lens.
Sync speed is the MAXIMUM speed that the shutter can run when using flash. At this speed or lower the shutter curtain on an SLR will be fully open when the flash fires. The actual duration of the flash is very short - thousanths of a second. Cameras with high sync speeds can cope with a wider range of subject matter when balancing daylight and flash.
I have attatched a graphic by way of illustration.
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