What is the "meditation on emptiness" in MN 121?

What is the "meditation on emptiness" in MN 121 ? What does "emptiness" refer to in this sutta? Also, what does "oneness dependent on the perception of ..." mean in this sutta? > “Indeed, Ānanda, you properly heard, learned, attended, and remembered > that. Now, as before, I usually practice the meditation on emptiness. > > Consider this stilt longhouse of Migāra’s mother. It’s empty of > elephants, cows, horses, and mares; of gold and money; and of > gatherings of men and women. There is only this that is not emptiness, > namely, the oneness dependent on the mendicant Saṅgha. In the same > way, a mendicant—ignoring the perception of the village and the > perception of people—focuses on the oneness dependent on the > perception of wilderness. Their mind becomes eager, confident, > settled, and decided in that perception of wilderness. They > understand: ‘Here there is no stress due to the perception of village > or the perception of people. There is only this modicum of stress, > namely the oneness dependent on the perception of wilderness.’ They > understand: ‘This field of perception is empty of the perception of > the village. It is empty of the perception of people. There is only > this that is not emptiness, namely the oneness dependent on the > perception of wilderness.’ And so they regard it as empty of what is > not there, but as to what remains they understand that it is present. > That’s how emptiness is born in them—genuine, undistorted, and pure. > > ...... > > Whatever ascetics and brahmins enter and remain in the pure, ultimate, > supreme emptiness—whether in the past, future, or present—all of them > enter and remain in this same pure, ultimate, supreme emptiness. So, > Ānanda, you should train like this: ‘We will enter and remain in the > pure, ultimate, supreme emptiness.’ That’s how you should train.”