The "doing" of fill-in flash is easier than the explanation. I'll try..
Separate the subject from the background, and take an exposure meter reading of both. If the background averages out at X exposure, and the subject reads that it needs considerably more than that, the ratio may be too great to obtain a working compromise. Expose for the background, and the subject will be excessively "dark"; expose for the subject, and you will blow out the background (common to backlighting); Set the exposure in the middle of the two, and the subject may be BOTH excessively dark AND the background will be "blown." It may be a good idea to "fill" the subject with light and reduce the difference between the two .. say to make the subject a half stop to a full stop darker than the background. One would meter the background, set the aperture and shutter speed accordingly; and set the flash unit to provide one-half - or so less light than indicated for a full exposure to the subject. That would be known as a "one-half stop fill."
Now ... every flash unit has a flash duration time. Usually an electronic flash duration is *very* short ... and the shortest times will be encountered at reduced power rather than at full blast. My Dynalites, for example, have a flash duration of 1/450th of a second at full power, deceasing to something like 1/10,000th second at minimum power. If the shutter speed is slower than the flash duration, some light will not get to the film. Not a disaster, but a consideration when determining exposure - that is why "Flash Meters" have a shutter speed setting - they will measure the amount of light received through a give "time window".
I've read through this five times. Hopefully it will make sense to someone else ..