This poem, like many that Herbert and his contemporaries wrote, is built around a conceit, an elaborate extended metaphor. Here, Herbert uses the tale of a tenant farmer asking his landlord for a favor to stand for the relationship between humanity and the Christian God.
Herbert's conceits were often gently funny and deadly serious at the same time, and this one is no exception. The speaker's tale of going to call at his landlord's "manor" and then finding that the guy is away on a business trip could sound totally ordinary, except for the fact that the "manor" is in "heaven" and the business trip stands in for the incarnation of Christ (that is, God taking on a human body and living on earth).
By matter-of-factly mixing the everyday and the divine, the poem both gives readers a nudge in the ribs—this definitely isn't just about a tenant and a landlord!—and makes the meaningful point that there is no separation between the everyday and the divine. The "redemption" the poem discusses is the stuff of daily life, and God offers it to everyone, farmers, lords, and "thieves" alike.
On a related note, the poem's conceit also helps to point out the difference between the way that people wield power and the way that God wields power. When the speaker, learning his landlord isn't at his heavenly "manor," goes to seek him on earth, he does what seems sensible based on how most "rich lord[s]" behave: he checks the fanciest places, the places where aristocrats of "great birth" hang out.
But this "lord" doesn't work like that. The speaker is shocked to find him among "thieves and murderers," and even more shocked when he answers the speaker's request before the speaker even asks him about it, immediately declaring, "Your suit is granted" (in other words, "You can have what you came to ask for"). With that, he promptly dies—on the cross, readers can assume—and it's through that death that the speaker's request is granted. God doesn't just offer people redemption, God pays for their redemption.
If Christ is a landlord, the poem's conceit thus suggests, he's a landlord unlike any on earth. Divine power, unlike human power, is humble, self-sacrificing, and boundlessly loving.