Chocolate is a food item which provides a small amount of nutrition, filling a colonist's hunger meter by 10% per unit consumed. Unlike most other foods, it also grants a bonus to Joy (Recreation) of 10% per unit.
The low nutritional value and rarity of chocolate makes it a poor choice to try to sustain a colony with, but its joy-boosting properties make it excellent for keeping colonists' mood up.
Colonists will eat no more than 4 units of chocolate at a time, and won't touch it on their own unless they are desperate for food. Manually directing troubled colonists to eat it when necessary is advised.
Chocolate can be acquired by planting cocoa trees, making chocolate good for selling or improving colonists' moods. They can also be purchased from bulk goods traders, or occasionally arrive in cargo pods.
In Beta 18 or earlier, chocolate cannot be produced by your colonists, and were exclusively a trade good.