This summary is not the rule. The rule text controls.
When a lawyer is working on a client's case, they cannot lie to other people about important facts or the law. This means the lawyer can't knowingly tell someone something false if it matters to the situation at hand.
The rule also covers staying silent when honesty requires speaking up. If a lawyer knows their silence would help a client commit fraud or a crime, the lawyer generally has to speak up and share the important information. The only exception is when another rule, Rule 1.6, requires the lawyer to keep that information confidential.
In short, lawyers must deal honestly with people outside their own client relationship. They can't actively lie, and in most cases, they can't stay quiet if doing so would help a client cheat or break the law.