It's one of the most quoted lines of St. Francis; it ends up in intercessory prayers and on holy cards for professions of vows. Francis's insight that his brothers were a gift from the Lord is an attitude Franciscans hold dear, and it informs our theological idea of who we are as an Order and a movement.
Sometimes I make fun of how we use the phrase; in the Testament of Francis it is only a dependent clause:
Et postquam Dominus dedit mihi de fratribus, nemo ostendebat mihi, quid deberem facere, sed ipse Altissimus revelavit mihi, quod deberem vivere secundum formam sancti Evangelii.
"And after the Lord gave me some brothers, nobody showed me [NB, not 'us'] what I had to do, but the Most High himself revealed to me that I [note: not 'we'] had to live according to the form of the holy Gospel." (Testament, 14)
For Francis, the primary kindness of God was the revelation of his vocation to live "according to the pattern" of the holy Gospel, a gift that came attendant on the gift of brothers. To live the gospel according to the Franciscan idiom requires a consciousness of sisters and brothers given to one as a gift.
It's something of which I am increasingly aware the further I get from the program of initial formation. All along the way, brothers have been given to me in various roles to help me along: friends on the natural level, companions on the 'inner way,' confessors, and wise elders who can straighten out my thinking. Even brothers who are enemies are a gift in the sense that they provide an opportunity to carry out Jesus' teachings on charity and a chance to refuse to participate in the cultures of gossip and detraction which can so poison a religious community.
This personal experience reminds me of two spiritual attitudes I must foster in myself. First, I must have the courage to go to the brothers who are given to me. When a suggestion in prayer tells me to call this brother, or that I should talk something over with another whose wisdom I trust, these are probably the promptings of the Holy Spirit working through the fraternity. To become willing to go to others for help when the Spirit suggests it is the greatest school of humility. (Note that this is also one of the reasons why we must be living lives of daily prayer.)
Second, I must note that God is probably using me in the same way. In the mysterious economies of grace, perhaps I am the brother that somebody else needs at some time in his vocation. Many times this will be hidden from me; the Spirit does this in order to shield us from the self-consciousness and vainglory that would get in the way. Therefore, I must be attentive and present to the brothers, because I never know what the Holy Spirit is doing through me, often in spite of me, and whether I know it or not.