Life is not a matter of holding good cards, but of playing a poor hand well.
Luck deals the cards, but the living is done by the player. The saying separates what is handed to us, such as family, health, talent, money and timing, from what we do with those things. Since the first part is mostly out of our hands, it suggests that judging a life by its cards misses the point. What counts is the quality of the play: patience, nerve, and a willingness to make something of modest material.
This is close to the Stoic outlook, where Epictetus argued that we should spend our energy on what is up to us and meet the rest calmly. It also fits Stevenson's own story, since he lived with fragile health yet kept writing and traveling. That said, the line is sometimes credited instead to the American humorist Josh Billings, so the attribution is not certain.
Today, imagine someone laid off in a weak job market. They cannot control the economy, but they can decide to update their skills, tell friends they are looking, and take a short-term contract to stay afloat. They are not pretending the hand is good, only playing it carefully.