"Sol could cast our light, even in the dark she would see. She lived deep in this grove, now she rests here in peace."
The Lost Grove is one of two new locations introduced to Ori and the Blind Forest by its Definitive Edition, the other being the Black Root Burrows. The Lost Grove is the former home of Naru and her father; their shelter still lies untouched at the bottom of the grove.
The Lost Grove cannot be accessed until its sister location, the Black Root Burrows, is completed, and even then Ori must have learned certain skills to progress. If Ori is not experienced enough, Sein will ask them to explore more of Nibel before returning and venturing into the Lost Grove.
The entrance to the Lost Grove is found deep within the Black Root Burrows. While the Black Root Burrows only requires Wall Jump for completion, the Lost Grove demands more experience from Ori, therefore a certain skillset must be available.
Early on in the grove, Ori learns that young Naru had formed a bond with two Guardian Spirits, Eki and Sol. But her father, concerned that his daughter was interacting with creatures of light while they themselves were lightless, ended the friendship before it could grow, separating them forever.
Not far from the memory, Ori comes across the Ancestral Tree of Sol, from which they learn Light Burst. This becomes useful later on, as Ori can cast the light upon Lanterns, which reveal hidden places.
At the very bottom of the Lost Grove, Naru's childhood home is found. An old memory plays out before Ori and Sein, recounting Naru and her father's peaceful life there. It reveals that Ori and Naru's pasts are nearly identical copies of each other, as it ends with Naru's father's death, and her permanent leave.
Sein reminds Ori that although they (Naru and her father) are gone from the forest, they cannot stop to grieve. Remembering them will let them live on.