What is the prohibition in the fourth commandment?

The core idea of the fourth commandment is to keep the Sabbath holy. This is more of a "do" than a "do not", but the rest of the commandment fills in some of the "do nots" to help people fulfill the "do". According to the commandment, the main thing to avoid doing is working on the Sabbath. This prohibition extended to all the members of the family and even guests or "strangers" in your house. The reasoning, which is included in the commandment, is that God made the world in six days, and rested on the seventh, and made it holy, so we should also rest on that day.

In The Bible this commandment was taken very seriously, and people were stoned for not following it. Gathering firewood was the first infraction after the Ten Commandments were given on Sinai, and the Sabbath-breaker was stoned for it. There are other similar examples in Scripture, but by Jesus' day, the priests and leaders had invented many additional restrictions to try to keep people from breaking the Sabbath. Jesus attempted to help people understand that these were against God's will. This is why he said things like "It is lawful to do good on the Sabbath." He was trying to clarify what the religious leaders had made confusing and unnecessarily complex. It isn't so much about what you shouldn't do as about what you should.