This summary is not the rule. The rule text controls.
This rule stops law firms and lawyers from making deals that limit a lawyer's ability to practice law in the future. For example, a law firm cannot make a lawyer sign an agreement saying they won't work as a lawyer in a certain city or won't take on certain kinds of clients if they ever leave the firm. The only exception is for retirement plans, where a lawyer might agree to stop practicing law in exchange for retirement benefits.
This rule also applies when lawyers settle a case for a client. A lawyer cannot agree to hold back from taking similar cases in the future as part of a settlement deal. For example, if a lawyer settles a lawsuit against a company, the company cannot ask that lawyer to promise they won't represent other people with similar claims against the same company later on.
The point of this rule is to protect people's ability to choose their own lawyer. If lawyers could be bought out of practicing in certain areas or against certain companies, it would become harder for regular people to find good legal help. This rule keeps the legal market open and makes sure lawyers stay free to represent whoever needs them.