This quote means that a laugh is an intense and unexpected expression of joy or happiness that is so strong that it bursts out in the kind of a smile. It is a physical indication of one's happiness that can not be contained.
"It scares you: all the noise, the rattling, the shaking. But the look on everybody's face when you're finished and packing, it's the best smile in the world; and there's nobody hurt, and the well's under control"
"Tears are sometimes an inappropriate response to death. When a life has been lived completely honestly, completely successfully, or just completely, the correct response to death's perfect punctuation mark is a smile"
"Don't write anything you can phone. Don't phone anything you can talk. Don't talk anything you can whisper. Don't whisper anything you can smile. Don't smile anything you can nod. Don't nod anything you can wink"