X is the symbol that denotes that a plant is a hybrid species. Hybridized fruits and vegetables are developed by cross pollinating two or more similar cultivars within one species or closely related species of the same genus. The resulting fruit or vegetable has the nutritional profile of both the parent food.
These are called F1 (first generation hybrids), because they are the direct product of a cross (X), for example the Pineberry which is a hybrid of Fragaria chiloensis and Fragaria virginiana (Fragaria chiloensis x Fragaria virginiana). Another F1 hybrid is courgette One Ball (see picture).