"The Flower" begins in the middle of an inner springtime—that is, a time of rebirth and renewal. The poem's speaker, astonished to find himself happy again after a time of deep sorrow, makes a grateful apostrophe to God, marveling at just how "sweet and clean" God's "returns" are.
The word "returns" here has multiple meanings. On the one hand, those "returns" might be gifts—answers to the speaker's prayers. On the other, they might be God's literal returns, times when God seems to come back after an absence. And perhaps both of these are true at once: God's return is an answer to the speaker's prayer.
Such "returns" feel, to the speaker, as joyful as early spring—and as restorative. Listen to the speaker's simile here:
[...] even as the flowers in spring;
To which, besides their own demean,
The late-past frosts tributes of pleasure bring.
In other words, God's presence is as much a delight to the speaker as the sight of new flowers. When flowers come out in spring, the speaker observes, they don't just beautify their own season (their "demean," or domain). They make even the recent "frosts" of winter look better in retrospect. Those frosts can't have been so bad if they led to these flowers.
These images of changing seasons thus evoke the speaker's sheer relief. If God's "returns" are like early spring, then the speaker must recently have been suffering through a long cold metaphorical winter—a time of emotional "frosts" when God seemed either not to be there, or not to be listening. Now that God is back, even that icy time seems warmed by "tributes of pleasure."