They refer to the species of the cannabis plant. Sativas are generally thought to be more uplifting and energizing, while indicas are thought to be more relaxing and sedating.
While shopping by species seems simple, research has shown that it’s not actually the species that shapes your high. It’s the chemical structure of the plant, mainly cannabinoids (like THC and CBD) and terpenes.
Popular sativa strains tend to have higher concentrations of THC (head high), while popular indica strains tend to have a more equal balance of THC (head high) and CBD (no head high, mainly therapeutic).
It’s not a hard rule, though. Some sativa can be just as balanced as indica, so it’s a good idea to learn how to read a label. It’s not as complicated as you may think. More on that below.