The sun's light beams in straight lines across the board from the token, and if your trees get touched by it, you get light points, which you can spent to plant more trees, or grow your existing ones. You'll also place the huge sun token along two sides of the board.
At the start of the game, you'll place two small trees in spaces near the edge of the hexagonal board, and you'll have a bank of more small trees, medium trees and large trees ready for later in the game.
