God said, "Keep holy the Sabbath," but does the Bible or Torah ever explicitly explain what that means?

Yes, it says that keeping the Sabbath means not doing your own thing on that day. Focus on God and drawing near to Him. Don't work, because we saw how Jesus threw out the people who were trying to sell stuff on the Sabbath, so it sure seems like He does not want us to work on that day. I keep the Sabbath, and I just rest and try to focus on God and worship Him.