Grow Forest is an ability unlocked after researching the Spiritualism tech. It allows players to grow forests on field tiles and crop tiles. It costs 5 stars to grow a forest. Forests can only be grown on tiles in your territory.
With this tech, it is possible to create forest tiles within your empire to build temples in. Other than that, this tech is only of any practical use if one has already researched farming and chivalry, which will allow one to burn down the forest after it is put in place to create crop on the tile, which then can be turned into a farm. Making a farm in this way will cost 15 resources in total.