Once when I was very young in religion, an illustrious friar came to give us a talk on holy poverty.
He explained it very simply: we are poor whether we like it or not. We are poor in our creatureliness. We can only be in one place. We can only do one thing at a time. We can't know everything, please everyone, or have everything. We're all limited in a million ways and ultimately subject to sickness, decay, and death.
Thus we all live in a state ontological poverty; it's just the lot of a creature to live in incompleteness.
So with this realization we can do one of two things. We can panic and try to make up for our lack by greedily amassing security and recognition and pleasure and flatterers, wrapping all these things around ourselves to try to mask our identity as poor creatures. We grasp and grasp, hoping to make up for the poverty within. At best we will fail to fool ourselves in this way and are led into misery. At worst we will succeed in fooling ourselves and are led into violence and moral poverty.
On the other hand, we can accept our creaturely poverty before God. Once we learn that no created thing will change this, we are free to use the things of this world without having to grasp at them and possess them as proper to ourselves. And that's what it means to live Franciscan poverty, the life of sine proprio. That's holy poverty.
And by the way, when you apply sine proprio to your relationships to other people, it's called chastity, and that goes for everyone, whether called to marriage, celibacy, or the single life.