You who have Christian backgrounds will recognize in that phrase as part the Christian beatitude,

"Blessed are the poor in spirit, for theirs is the kingdom of God."

Without dealing much with the Christian terminology, that verse expresses something very germane to Buddhist teaching. When we come to see in very real terms that we do not own anything, we  become poor in spirit whether or not we still manage things useful to us in the flux of life.

The Buddhist teaching of non-attachment is directly related to the realization that there is not a self that can own things,  that all forms are only a transient flux, never still for a moment -- that there is no possibility of stemming the flow and grasping anything permanently. Life itself is a rental of sorts -- even your own body and mind. And if you cannot really possess those things, you cannot possess anything at all. If you really see that, the weariness of your unrelenting grip to own things is replaced by relaxation of the weary holding on in an attempt to grasp and possess the flow. One is reminded of a paraphrased quote from Ram Dass,

"You realize that you are falling, and that you have no parachute. And then you relax; you have just realized their is no ground."

The ability to be "poor in spirit" is a great freedom, in particular for the lay Buddhist who does often have residual resource responsibility to family and dependents. It is possible, even as a lay Buddhist, to relinquish the grip on things -- to understand deeply that you do not own, you have never owned, and will never own anything. So you might as well accept and enjoy the flow without being anxious about its nature.