This summary is not the rule. The rule text controls.
Lawyers and law firms cannot make deals that stop a lawyer from practicing law in the future. For example, a law firm cannot make an employee or partner sign an agreement saying they will give up clients or stop practicing in a certain area if they ever leave the firm. The only exception is for retirement plans, where a lawyer can agree to limit their practice in exchange for retirement benefits.
This rule also applies when settling lawsuits. If a lawyer is helping settle a case between two private parties, they cannot agree to terms that would stop them (or another lawyer) from taking similar cases in the future. For instance, a company being sued cannot offer to settle a case only if the plaintiff's lawyer agrees never to sue that company again on behalf of other clients.
The idea behind this rule is to protect people's ability to choose their own lawyer. If lawyers could be bought out of practicing certain kinds of cases, it would become harder for future clients to find good legal help. This rule keeps the legal system open and fair for everyone.