There's a famine in the center of each sweet; There's a surfeit in the larder of the sour; And the longest pilgrimage that ever was— May be less than Mecca half way down a flower.
The poem is a bee's-eye view of flowers, built on a few neat reversals. The sweet ones, we're told, are empty at the center: a "famine" where you'd expect plenty. The sour ones, meanwhile, have a full pantry. So the flowers that seem most appealing to us may offer a bee almost nothing, while the plain or sharp ones hold the real reward.
The last two lines shift the scale. For a bee, a journey down into a single blossom is as great as a pilgrimage, perhaps even greater than the trek to Mecca. The poem asks us to see something huge inside something tiny, and to treat a bee's daily work as a kind of sacred travel.
The mood is playful but a little awed. The speaker sounds amused and quietly clever, enjoying the paradoxes, yet there's real respect for the bees' patient, devoted labor. Small things, the poem suggests, can carry enormous weight.