The Midnight Burrows are a somewhat secret area in Ori and the Will of the Wisps, and you can access them from the bottom of the Inkwater Marsh, where you'll find Tokk standing out front. He'll ask you to help him find a stone tablet lost in the Burrows, but say that can't figure out how to read the music notation that opens the way.
The stones behind Tokk are that notation. Their sizes correspond to the length of each of the three flower bells to the right. Not the length of the vine, mind you, the length of the bell itself. So the tall stones correspond to the left bell, the short stones to the center bell, and the medium stones to the right bell.
The order to play the bells in is: Left, Center, Right, Right, Left, Center, Left.
You can strike the bells by using the Bash ability, kicking things off by hopping on the jump pad and air-dashing to the left side. Once you complete the sequence, the way will open. See how to do it in the video above (also on YouTube (opens in new tab)), and below is the location of the entrance on the map.