I see and hear lots of questions about “guide numbers.”
First off, guide number aren’t used for studio flashes (packs and monoblocks) because which modifiers (reflectors, snoots, soft boxes, umbrellas, …) you use affects it enough to render any guide number useless . Read on and you’ll understand.
I encounter quite a few hobbyists who don’t know what a “stop” is. A stop, in photographic terms, is a relative value for exposure to light, or a change in exposure by a factor of two, or +/-1 EV.
Got it? No? Okay then, if I change my exposure by one stop, I’m either letting in twice as much light, or half as much (depending on which way I’m adjusting). There are three ways I can adjust this on a modern SLR and each way effects other aspects of the photo. Understanding these other effects is important to controlling your image.