This area is known as the Deep Woods
Link first arrives here directly after finding all the missing Kikwis for the Kikwi Elder that were actually hiding around the woods. After doing so, the elder will give Link the Slingshot, telling him that Zelda went into the Deep Woods. Link now has access to the Deep Woods, as the Slingshot is required. Upon arrival, he will find tons of enemies guarding the area, including Bokoblin and Deku Baba.