This is a technique developed by Hashirama Senju, founder of Konohagakure. The user forces trees to grow on any surface, easily creating a dense forest anywhere they choose. With this technique, a small plant can grow into a forest in an instant. By generating chakra, the user manoeuvres it as they see fit for attack and defence, and furthermore, this omnipotent technique even allows them to capture the enemy at the same time. With their tremendous life force, the trees can pierce through earthen walls, and extend their branches at their prey in an instant.
In Japanese, "sea of trees" (樹海, jukai) is an idiom referring to a large expanse of dense woodland. The First's technique changed the last kanji from sea (海) to world (界), probably to reinforce the greatness of the forest created.