"This Spirit Gate blocks us... They were built for protection, before the days of decay."
"The required Keystones must be nearby. To reach the Spirit Tree, we must pass the Spirit Caverns that lie beyond this gate. "
The Spirit Gates are special stone doors, constructed to protect certain areas of Nibel. In the general gameplay of Ori and the Blind Forest, however, these doors serve as obstacles that must be unlocked to access more of the game's world and enable progression.
Spirit Gates require Keystones to be opened, either two or four. There is also a small slot above the Keystone holes in which Sein must go, but this is not an issue because Sein is with Ori almost the entire game (the very beginning is an exception to this).