This summary is not the rule. The rule text controls.
Lawyers are not allowed to sign agreements that limit their ability to practice law in the future. For example, a law firm cannot make an employee or partner promise to stop practicing law in a certain area, or to stop representing certain types of clients, after they leave the firm. The only exception is for retirement plans, where a lawyer can agree to give up practicing law in exchange for retirement benefits.
This rule also applies to settling lawsuits. When a case settles, the lawyers involved cannot agree to restrict one side's lawyer from taking similar cases in the future, unless a judge or government agency specifically approves it. For example, a company being sued cannot offer to settle a case only if the other side's lawyer agrees never to sue that company again.
The main point of this rule is to protect people's ability to choose their own lawyer. If lawyers could be bought out of practicing law, it would become harder for regular people to find good legal help, especially lawyers who have experience against powerful companies or in certain kinds of cases. This rule keeps the legal market open and makes sure clients keep their freedom to pick who represents them.