This summary is not the rule. The rule text controls.
Lawyers have to be honest when they're dealing with people on behalf of a client, whether that's the other side in a case, a witness, or anyone else involved. They cannot knowingly lie about something important, whether it's a fact or a point of law.
There's also a duty to speak up in certain situations. If staying quiet would help a client commit fraud or a crime, the lawyer generally has to say something to prevent that. The main exception is when speaking up would mean revealing information the lawyer is required to keep confidential under the rules about client confidentiality.
In short, this rule stops lawyers from lying to get an advantage for their client, and it also stops them from using silence as a way to help a client cheat or break the law. It balances honesty toward others with the lawyer's duty to protect client secrets, but honesty wins out when a client is using the lawyer to commit fraud.