This will be a bit of a simplification, so apologies in advance to any taxonomists!
The scientific names of plants, which we as gardeners frequently use, are based on the “family tree” of evolution, so it reflects our current knowledge of what descended from what. This has practical uses, as closely related plants will have similar flower shapes, similar toxins, maybe similar growth requirements.
For our purposes, i’ll start halfway down the tree with Families - Rose family, Buttercup family and so on. The scientific family name always ends in aceae, eg Rosaceae.
Next level down is Genus - Geranium, Fuchsia, Cornus etc.
The basic building block is the species, and has a double name to indicate the genus, and then a second to specify which species, eg Geranium sanguineum. Often the species name is descriptive, in this case “bloody” (cf sanguinary) - in English Bloody Cranesbill. The species name doesn’t have a capital letter, it’s wrong to write “Geranium Sanguineum”.
If one species can cross breed with another (either naturally or deliberately by a breeder) to produce a hybrid, this is shown by a x, eg Hyacinthoides x massartiana, the hybrid between English and Spanish bluebell. Rarely, two genera can hybridise, eg xFatshedera, the hybrid between Fatsia and Hedera.
Often, within a species, different varieties can arise or be created by breeders, not different enough to be different species, but distinct enough to be treasured for better flowers, earlier fruit, disease resistance or whatever. Their names are in quotes and can have capitals. Eg Rosa moysii ‘Geranium’, or Geranium ‘Roseanne’.
And cabbage, sprouts, broccoli, kohl rabies are all vanities of Brassica oleracea, eg B. olercea var ‘sabuada’, Savoy Kale.