I just give you some and hope other can give you more.
Din or ASA is a grading of film sensitivity by different countrty. Din, if I am not wrong is Europe grading and ASA is American grading. Din 21 is the same as ASA 100.
Some call it film speed - speed of accepting the light.
Aperture and shutter speed is a combination, called exposure, to allow proper amount of light to create an images on the film.
For example, on bright cloudy day, aperture of f16 at the shutter speed of 1/250 secs can be changed to an aperture of f8 at the shutter speed of 1/500 secs - either exposure will be accurate. ( if you measure it using your camera of light meter )
This comibination is also call exposure value - EV for short.
I hope it give you some idea to digest first.
I hope other can continue to add on to assist you.
Usually there should be a table at the back of the flash to show distance,
ASA. and aperture to use.
Eg: At 10 feet, ASA 100 the F-Stop usually is F5.6 - all this is for a normal flash with GN of 30.
Base on Inverse law of light, every change of distance will change the power of light. Eg: if the distance is cut by 2 feet to 8 feet, your aperture should be f8. Vise versa if it add another 2 feet, the f stop should be f4.
Another method is using light meter. use a measuring tape and measure the distance. Start with 10 feet distance and fire the flash to test the f stop. Use every 2 feet in change of distance.
Is your flash is purely a TTL flash for the camera made - eg nikon or canon?
Is that is the case, perhaps it has no flash distance table at all.
Then you need to change to a higher model flash of your camera made( eg nikon or canon).