Fire was needed in building the Mishkan for heating the vats in which the dyes were made and for smelting the metal needed for the various utensils.
Unlike most melachos, mavir is explicitly stated in the Torah (“You shall not light a fire in any of your dwelling places on the day of Shabbos,” Exodus 35:3). Because of mavir, we may not light a fire on Shabbos, nor can we feed an existing flame. We may, however, benefit from fire that was made before Shabbos. For this reason, we light Shabbos candles, to counter the heretical interpretation that would suggest that we are meant to sit in the dark all Shabbos.
Aside from keeping us from lighting or turning up the fire on the stove, mavir impacts a number of other activities that on the surface may not be apparent that they involve using fire. The first of these is driving, though upon a moment’s reflection, it becomes clear just how much driving involves mavir. You place your key in the ignition (which, as the name suggests, ignites), spark plugs spark, you burn fuel – driving is all about mavir!
Mavir is also the reason we don’t shower or bathe on Shabbos – as a preventive measure to avoid coming to heat the water.
The most obvious application of mavir in our society was a non-issue little more than a century ago: we don’t use electricity on Shabbos. For the purpose of mavir, electricity is considered the same as fire. (The extent to which the use of electricity on Shabbos violates a Biblical or a rabbinic prohibition is beyond our scope. Suffice it to say that certain leniencies may be relied upon in emergency situations; consult your halachic advisor.) Because of mavir, we do not use the phone on Shabbos, we suppress the light bulbs in our refrigerators and we set lamps on timers. (The idea of a “Shabbos elevator” that runs automatically is more complicated than it might seem, as a passenger’s weight might directly affect the amount of energy generated. Many poskim limit their use to the elderly or infirm. Again, consult with your own rav for guidance in this area.)
This is just an introduction to the concepts of the melacha of mavir; it is not a substitute for a full study of the halachos.