Judaism’s definition of work in reference to Shabbat bears little resemblance to the 9-5 definition of the term that we are accustomed to. The “work” that is prohibited on Shabbat is what the Torah calls “melachah.” Melachah is defined as melechet machshevet, skilled work. Thirty-nine primary forms of work are forbidden on Shabbos because Hashem wants us to truly enjoy a complete day of total leisure.
Rabbi Samson Raphael Hirsch explains that the “work” prohibited on Shabbat is the creative development of the physical world:
The word “melachah” appears two hundred times in Scripture and never once refers to physical labor. The melachah which is forbidden on Shabbat is conceived as the execution of an intelligent purpose by the practical skill of man: i.e. any production, creation, or transforming an object for human purposes; but not physical exertion. Even if you tired yourself out the whole day, as long as you produced nothing within the meaning of the term melachah, as long as your activity has not been a constructive exercise of your intelligence, you have produced no melachah. On the other hand, if you have engendered, without the slightest exertion, even the smallest change in an object for human purposes, then you have profaned the Shabbat, and undermined your calling as a Jew …
These 39 forms of labor are systematic and methodical so that we can study and review them during every Shabbos meal when we learn the system. Thus, we should review these 39 diamonds each week to recognize and enjoy these gifts from Hashem.