The guide numbers are to do with the light emitted by the flash at full power. Based upon the amount of light , over a distance, using ISO/ASA 100 film . A bit daunting . But in essence, The higher the guide number, the brighter the flash. Typically a macro flash will be around 8 to 14. An on camera "Pop up" will be 10 to 15 ish. And the extra add on flash gun of which you speak will be at least 20 up to 60 odd, sometimes even more.
The guide numbers may be found, usually in the hand book, and often in the title of the flash (eg, Minolta Auto 360 PX, has a guide number of 36. A Canon speed lite 580 EX, Has a guide number of 58. A Konica Minolta P-Flash 5600HS(D) has a guide of 56. etc ). Slightly at odds with the norm the quantray has a guide number around the 100 mark. It also has all the bits needed to bounce and to work fully automatically (Through The Lens (TTL) metering etc for the specified camera that it was intended for.
FMWs post is a delightful image, and what he says has a certain ring of truth about it. But using the walk before you run technique. I would suggest that you get used to the camera and flash for a while before you go into remote triggered off camera stuff. It will be better than on the camera . but is a lot more complex. Start by using the flash on the camera, At first the camera will work out all the sums for you. As you get used to it you will start to do it for yourself. Then, as suggested, bounce it off a (white) ceiling (or it will pick up the colour of what ever surface it is bounced off, And remember the power of the flash required to cover not just the camera to subject. but the distance from the flash to the ceiling and the ceiling to the subject and back to the lens, not of course forgetting the inverse square law, where light diminishes as it spreads out over distance). Then you can take it off the camera using a dedicated lead (quite expensive, but well worth the money). But like I said lets just walk before running, have fun, enjoy and learn the basics first.