Lets take Dogens advice and try to examine the meaning of this:
It is not only that there is water in the world, but there is a world in water. It is not just in water. There is also a world of sentient beings in clouds. There is a world of sentient beings in the air. There is a world of sentient beings in fire. There is a world of sentient beings on earth. There is a world of sentient beings in the phenomenal world. There is a world of sentient beings in a blade of grass. There is a world of sentient beings in one staff.
Wherever there is a world of sentient beings, there is a world of buddha ancestors. You should thoroughly examine the meaning of this.
Excerpted from Essential Zen by Kazuaki Tanahashi & Tehsho David Schneider.
"Worlds" of sentient beings are also called the 18 Sense Realms. The entire "world" we assume to be external to us is actually only present within the limitations of our senses. That is what is meant by the world of sentient beings in all these phenomena, because these phenomena are only fabricated objects of the 18 Sense Realms.
"Wherever there is a world of sentient beings, there is a world of Buddha ancestors" means what is also a line from the Shurangama Sutra; "The Sages and the Ordinary People's path are not two."
That means; seeing, hearing, smelling, tasting, touching, and thinking. Ordinary people conceptualize and grasp at these experiences, creating falseness in reality. A Buddha does not follow their thinking, but just sees, just hears, just smells, etc..
So the Buddha says in the Avatamsaka Sutra; "I see that all living beings have the virtuous qualities of the Tathagata's wisdom, yet due to conceptualizing the unreal and grasping, they are unable to realize it."
As in Zen, Bodhidharma states in his Bloodstream Sermon; "Whoever sees his nature is a Buddha; whoever doesn't is a mortal."