Arcanite Grass is a block for planting crops from the Dungeon of Arcana. It is important because these crops will grow on Arcanite Grass and nowhere else.
In order to obtain Arcanite Grass, a player must use the Staff of Enrichment on Arcanite Dirt. The Arcanite Dirt, in turn, is obtained by trading Arcanium with Leorna. Leorna and Arcanium can both be found in the Dungeon of Arcana.
Arcanite Grass is mainly used to plant seeds that are purchased from Leorna, since these plants will only grow on Arcanite Grass.