In a
true fruit/
botanical fruit, the fruit wall (
pericarp) develops primarily from the
ovary.
In an accessory fruit, significant edible tissue comes from other floral structures, such as the receptacle,
hypanthium, or floral tube.
The apple is a classic example of an accessory fruit because much of what you eat is derived from the hypanthium/receptacle rather than simply the ovary wall.
The strawberry is another famous example: the fleshy red part is largely receptacle tissue, while the little “seeds” on the surface are actually individual fruits (i.e.
achenes).