This is the Jewish day of rest. It lasts from sundown on Friday to sundown on Saturday. Observing Jews – known as “shomer Shabbat” – are prohibited from doing any work during this time.
The definition of “work” comes from 39 prohibited categories. Many of things are historic jobs performed by craftsman, like grinding, smoothing, carrying, etc. Here’s a detailed list.
That page is an excerpt from a book, and it states:
the definition of such work is any act where man demonstrates his mastery over nature
There are other traditions associated with Shabbat, such as prayers, the lighting of candles, communal meals, and the eating of a special bread called Challah.
Shabbat comes from an interpretation of the fourth commandment:
Remember the sabbath day, to keep it holy
Both “sabbath” and “Shabbat” come from a Hebrew word meaning “rest.”
The word is pronounced how it’s written, but I found differing opinions on which syllable gets the emphasis. Either way seems to be acceptable. It’s also written (and pronounced) “Shabbos.” It turns out this is mostly by the Ashkenazi Jew population.