After you completed the Forbidden Forest, most of the Koroks flew to different islands in the Great Sea to check on the baby deku trees. All of them are withered and wilting. Link can bring them back to health with the help of some forest water. Here is how.
In the Forest Haven, you can collect some water in a bottle. This water is special forest water with magical healing properties. However, 20 minutes after you collect the water, it turns back into plain old water with no magical powers.
Your task is to go around to the withered baby deku trees and pour the forest water on them before the water turns back into regular water. If you succeed, you will receive a piece of heart. If you don't make it in time, the trees will wilt again and you will have to try again.
You will have an unlimited supply of the forest water, at least until it turns back into regular water, so if you accidentally pour it out on the wrong spot, you can just try again.
Here is how I recommend watering all of the trees.
First, warp to the Forest Haven and set the wind to east. Go inside of the Forest Haven and use an empty bottle to get some forest water from the inner area (the water outside is just normal water.) Run outside, let the waterfalls take you down, get into your boat, and sail one square east to Cliff Plateau.