Plant it to grow One-Way Block. This block can only be moved through in the direction the arrow is pointing. From the other direction, it might as well be a wall!
One-Way Block Seed price in Growtopia: sells for about 4 WL (median of 1 offers from community Discords, past 30 days). Live price chart and the latest buy and sell offers.
Recently sold at: HEDCRIME1