This summary is not the rule. The rule text controls.
Lawyers cannot agree to limit their own right to practice law in the future, and they cannot ask other lawyers to accept such limits either. For example, a law firm cannot make an employee sign a contract saying that if they ever quit or get fired, they cannot go work for a competing firm or start their own practice in the same area. The only exception is for retirement agreements, which can include reasonable limits on practicing law after someone retires and starts collecting retirement benefits.
This rule also applies when lawyers settle lawsuits for their clients. A lawyer cannot agree, as part of settling a case, that they personally will stop representing clients with similar claims against the other side in the future. For instance, if a lawyer settles a case against a company, the company cannot require that lawyer to promise they will never take on another client suing that same company.
The idea behind this rule is to protect people's ability to choose their own lawyer. If lawyers could be bought out of practicing in certain areas, it would become harder for future clients to find good legal help, especially against powerful opponents who have the money to make these kinds of deals.