This summary is not the rule. The rule text controls.
When a lawyer is working on a client's case, they cannot lie about important facts or legal points to other people involved, like the other side's lawyer, witnesses, or anyone else outside the case. This means a lawyer cannot knowingly tell someone something false if it matters to the case.
The rule also says lawyers cannot stay silent when speaking up is the only way to stop a client from committing fraud or a crime. If a lawyer knows their silence would help a client do something illegal or dishonest, they usually have to speak up. The one exception is when another rule, Rule 1.6, requires the lawyer to keep client information confidential. In that situation, confidentiality rules can override the duty to disclose.
Breaking this rule is serious. A lawyer who violates it can face punishment as severe as losing their license to practice law.