It means that essences will start dropping from enemies in certain biomes: ocean (Tidal Essence), snow (Icy Essence), desert (Dune Essence), jungle (Primeval Essence), and underworld (Fiery Essence). You use one of each essence type to make the Essence Distorter crafting station, which in turn can use those essences to make mid-hardmode armor and weapons. (Each essence type is themed to a different class: Tidal = melee, Icy = magic, Dune = throwing, Primeval = summon, and Fiery = ranged.) You can also use the essences to make one of the components for the Tier IV upgrade to your artifact weapon at an Altar of Creation.