This summary is not the rule. The rule text controls.
When a lawyer is working on behalf of a client, they can't lie about important facts or important points of law to other people involved in the case. This means the lawyer must be honest when talking to the other side, witnesses, or anyone else connected to the matter.
There's also a second part to this rule. If staying silent would help a client commit fraud or a crime, the lawyer usually has to speak up and share the important facts instead of hiding them. The main exception is when another rule, Rule 1.6, requires the lawyer to keep client information confidential. In that case, the duty to protect client secrets can override the duty to disclose.
In simple terms, this rule stops lawyers from lying to get an advantage for their clients. It also stops them from staying quiet in a way that helps a client cheat or break the law, unless client confidentiality rules say otherwise.