All tequila must be distilled from the Blue Weber type of agave plant, but for tequila to be considered premium tequila, the plant must be grown at higher elevations.
At more soaring heights, where temperatures are cooler, agave is packed with minerals and nutrients. It's dense in flavors and aromas, so distillers don't need any additives.
Premium tequila is defined by a lack of those additives. By law, tequila must be made from at least 51% agave, but the other 49% can be filled with additives for flavoring.
These additives can dilute the tequila's purity and lead to nasty hangovers. Conversely, tequilas considered premium feature higher percentages of agave, up to 100%.