Here's the way I understand it:
In the ten commandments we must observe the sabbath day and we shall not do any work or allow anyone around us to do any work.
Okay, but why do those crazy Jews refrain from driving and flipping light switches?
The building of the tabernacle requires 39 actions, types of work. And that serves for Judaism as the set of things you can't do on shabbat. It includes: making fire, completing tasks, carrying things, and writing. And so to this day observent Jews do not write or light fire or anything that smacks of those 39 things.
As usually there are a lot of talmudic expansions of what a Jew can and cannot do, but those are very complicated to detmermine and are often self contradictory.