What and why is "perception of the dimension of nothingness"?

This is a paragraph of [Cula-sunnata sutta](https://www.accesstoinsight.org/tipitaka/mn/mn.121.than.html) ; > **(Nothingness)** > > "Further, Ananda, the monk — not attending to the perception of the dimension of the infinitude of space, not attending to the perception of the dimension of the infinitude of consciousness — attends to the singleness based on the perception of the dimension of nothingness. His mind takes pleasure, finds satisfaction, settles, & indulges in its perception of the dimension of nothingness. > > "He discerns that 'Whatever disturbances that would exist based on the perception of the dimension of the infinitude of space are not present. Whatever disturbances that would exist based on the perception of the dimension of the infinitude of consciousness are not present. There is only this modicum of disturbance: the singleness based on the perception of the dimension of nothingness.' He discerns that 'This mode of perception is empty of the perception of the dimension of the infinitude of space. This mode of perception is empty of the perception of the dimension of the infinitude of consciousness. There is only this non-emptiness: the singleness based on the perception of the dimension of nothingness.' Thus he regards it as empty of whatever is not there. Whatever remains, he discerns as present: 'There is this.' And so this, his entry into emptiness, accords with actuality, is undistorted in meaning, & pure. This sutta is little confused to me. What is the purpose of this sutta? What is the meaning of, "His mind takes pleasure, finds satisfaction, settles, & indulges in its perception of the dimension of nothingness"? Can some one explain this sutta in simplified way?