Okay, I'll try.
When you expose film in "normal" circumstances, there is a reciprocal relationship between the intensity of the light reaching the film (determined by the ambient light intensity, and the aperture set on the lens) and the length of the exposure (shutter speed).
If the aperture is adjusted by one stop, you can get the same result by changing the length of the exposure by the reciprocal of one stop.
Same for adjustments of two, three, four or more stops.
In other words, reciprocity of light intensity and exposure is maintained.
However, if you start working outside of the "normal" range, the reciprocity starts to fail.
What constitutes the "normal" range will vary a bit between films.
When the light is so dim that the necessary reciprocal exposure time is very long - lets use two minutes as an example - the film responds less than when you are working within the normal range, so you need to add more light to the film to get the same result. You can do that by either changing (increasing) the ambient light, changing the aperture, or by increasing the exposure time even more. The guides to how much you need to increase the exposure time are a bit imprecise, because this varies with light conditions (contrasty vs not), subject brightness range and film type. Basically, you use the guides, you bracket, and you apply experience as you acquire it.
Just to be clear, although we tend to think of reciprocity failure as being linked to longer exposures, it is actually caused by the low level of light that necessitates those long exposures.
You may also encounter reciprocity failure with extremely short exposures (usually when using electronic flash at the boundaries of their appropriate operation) but I wouldn't worry about that for now.