Rulemaking is when a group of people with some sort of authority establishes regulations that are designed to influence how another group of people behaves.
In the context of the law, the Supreme Court participates in rulemaking when it creates procedures and standards for the lower courts to follow.
Generally, when rulemaking occurs, the group that is making the rules must have the power to enforce these rules. Otherwise, the rules will be meaningless.
The Supreme Court gets its power from the United States Constitution. The rules that the Supreme Court makes are upheld by the federal government. However, when the Supreme Court makes a new rule, it cannot create one that is in violation of the Constitution. For example, the Supreme Court could not suddenly decide to take away free speech.