poor of spirit means that in spirit you know you possess no possessions truly. everything is owed to God. you're just temporarily taking care of some of Gods possessions and he's going to come soon and see what you did with his possessions (see numerous parables).
Also:
Aug., City of God, book 19, ch. 1: The chief good is the only motive of philosophical enquiry; but whatever confers blessedness, that is the chief good; therefore He begins, “Blessed are the poor in spirit.”
Aug., de Serm. in Mont., i, 1: Augmentation of ‘spirit’ generally implies insolence and pride. For in common speech the proud are said to have a great spirit, and rightly - for wind is a spirit, and who does not know that we say of proud men that they are ‘swollen,’ ‘puffed up.’ Here therefore by “poor in spirit” are rightly understood ‘lowly,’ ‘fearing God,’ not having a puffed up spirit.
Chrys.: Or, He here calls all loftiness of soul and temper spirit; for as there are many humble against their will, constrained by their outward condition, they have no praise; the blessing is on those who humble themselves by their own choice. Thus He begins at once at the root, pulling up pride which is the root and source of all evil, setting up as its opposite humility as a firm foundation. If this be well laid, other virtues may be firmly built thereon; if that be sapped, whatever good you gather upon it perishes. [p. 148]
Pseudo-Chrys.: “Blessed are the poor in spirit,” [ed. note, a: The Bened. ed. reads ‘beati egeni’ - and has this marginal note, ‘Hinc sequitur hune Graece non scripsisse’ - but S. Thos. reads ‘beati πτωχοι ptochi;’ it may be remarked moreover that the author follows the order of verses 4 and 5 according to the Greek; all the Latin Fathers (with the single exception of Hilary on Ps. 118) following the order of the Vulgate.] or, according to the literal rendering of the Greek, ‘they who beg,’ that the humble may learn that they should be ever begging at God’s almshouse. For there are many naturally humble and not of faith, who do not knock at God’s almshouse; but they alone are humble who are so of faith.
Chrys.: Or, the poor in spirit may be those who fear and tremble at God’s commandments, whom the Lord by the Prophet Isaiah commends. Though why more than simply humble? Of the humble there may be in this place but few, in that again an abundance.
Aug.: The proud seek an earthly kingdom, of the humble only is the kingdom of Heaven.
Pseudo-Chrys.: For as all other vices, but chiefly pride, casts down to hell; so all other virtues, but chiefly humility, conduct to Heaven; it is proper that he that humbles himself should be exalted.
Jerome: The “poor in spirit” are those who embrace a voluntary poverty for the sake of the Holy Spirit.
Ambrose, de Officiis, i, 16: In the eye of Heaven blessedness begins there where misery begins in human estimation.
Gloss. interlin.: The riches of Heaven are suitably promised to those who at this present are in poverty.
Especially if you have a question about the Gospels, always refer to the Catena Aurea (the Golden Chain). While it was compiled by St. Thomas Aquinas, it's all just early church fathers. It's extremely helpful for understanding the Gospels.
https://dhspriory.org/thomas/
https://dhspriory.org/thomas/english/CAMatthew.htm#5
Hope this helps, God bless.