It is to come off from all confidence in our own righteousness and strength, that we may depend only upon the merit of Christ for our justification, and the spirit and grace of Christ for our sanctification. That broken and contrite spirit with which the publican cried for mercy to a poor sinner, is that poverty of spirit. We must call ourselves poor, because always in want of God’s grace, always begging at God’s door, always hanging on in his house.
Now,

This poverty in spirit is put first among the Christian graces. The philosophers did not reckon humility among their moral virtues, but Christ puts it first. Self-denial is the first lesson to be learned in his school, and poverty of spirit entitled to the first beatitude. The foundation of all other graces is laid in humility. Those who would build high must begin low; and it is an excellent preparative for the entrance of gospel-grace into the soul; it fits the soil to receive the seed. Those who are weary and heavy laden, are the poor in spirit, and they shall find rest with Christ. – Matthew Henry, Commentary on Matthew 5