Finding The Heart of the Woods is one of the three main objectives you'll have to complete in Hollow Knight: Silksong's later Act 3 section. After you emerge from the abyss with your new spectacular Silk Soar ability, the snail shamans will put you to work again, hunting down three ancient hearts from across Pharloom.
The Old Hearts sees you heading to three previously uncharted areas, and finding the entrance to each is actually rather tricky. The Heart of the Woods, especially, has a somewhat nonsensical entrance you could theoretically spend hours trying to find. That, or you could check this guide and be on your merry way towards the area's challenging bosses instead.
