[Dabhand16]

The guide number is what you use to manually calculate aperture settings. There ought to be a chart in the documentation of your flash as it varies with the ASA (ISO) setting, and the only source of this information is the flash manufacturer. It is a way of expressing the power of a flash. The in-built pop up flashes in most dSLR's will have a GN of about 12

Typically a decent flash will have a GN of about 56 @ 100 ASA. Aperture is GN divided by distance, so in the example here, if you were shooting at a distance of 10 feet, (or if the distance the light had to travel to your subject was 10 feet - if using bounce), the aperture you would set is f5.6.

Unless you are going to use manual settings, I would not worry too much about GNs. Modern kit will control the power and duration of the flash to achieve good exposures - better than the set and hope + bracketing of manual methods.

Current flash technology is more about controlling one or more flashguns to light a subject, or to provide fill light with fine tuning being achieved by exposure compensation from the camera's meter or by changing the power and/or position of one or more of the lights.