Ambrosia is a raw drug (not a vegetarian food) that can be harvested only after an ambrosia sprout event (also available through trade), but cannot be grown by the player. It provides a +5 mood buff, +50% to joy and is slightly addictive.
Addiction to ambrosia is trivial compared to other drugs; it takes only 10 days before full recovery, and does not impair functioning; it only causes a -10 mood debuff as that pawn craves more ambrosia.
It is also somewhat rare to get an ambrosia addiction, as each fruit only has a 1% chance to give one.
It is harvested from ambrosia bushes. They can be harvested multiple times before dying, each time yielding 4 fruit when fully grown.
A good way to harvest them is to draw a growing zone over them, but forbid sowing. Colonists will then automatically harvest the bushes when they are fully grown.