The central message Voltaire conveys in Candide is that all is not for "the best in the best of all possible worlds." The book satirizes and debunks that philosophy, which had gained traction in the mid-eighteenth century (when Voltaire wrote this work). Voltaire shows that world is a miserable, corrupt place for the people who actually encounter it and don't simply read about it safely from afar. Candide gets caught up in a series of horrible adventures that are so over-the-top terrible they become laugh-out-loud funny. To some extent, a reader doesn't know whether to laugh or cry: for example, when Candide's beloved, Cunégonde, explains, matter-of-factly, that it is possible to survive rape and disembowelment, as she has done.
Voltaire shows that the people who are supposed to make the world a better place, such as the clergy, participate in spreading the misery: Cunégonde is shared as a prostitute, for example, between a Jewish merchant and a Grand Inquisitor (Catholic priest).
When Candide does arrive in the New World, he finds a rational, Edenic society in El Dorado: in this way, Voltaire shows the world could become a better place, but sets this fine land far from Europe and its depravities.
In the end, Candide learns it is better to turn away from the wide world and "cultivate one's garden." Ours is not the best of all possible worlds, but by retreating, one can fashion a peaceful life for oneself: that is the central message of...