Hesperidia are a type of fruit, specifically a type of berry. As you can hopefully guess from the name, it is just a technical, botanical classification.
Hesperidia are basically just citrus fruits like oranges and lemons. A hesperidium is distinguished from other fruits by its thick spongy rind and by the fact that the different sections of its ovary are filled with juice sacs. I believe these different sections are caused by several ovaries joining together into one early in the fruit's development.