This poem, as the title implies, is ultimately concerned with virtue and the idea of a virtuous soul which will survive forever and will not "give" as earthly beautiful things do. In order to better establish this contrast, however, he begins with three stanzas in which he describes the way that transitory "sweets" on earth all succumb, ultimately, to decay as part of the natural order of things.
In the first stanza, he describes the day, personifying both it as the "bridal of the earth and sky" and the dew as its mourner, who will "weep" for it when evening comes and it is no more.
In the second stanza, he points to the transience of the rose, which is rooted for its entire life in the same place that will eventually become its grave. A rose may be beautiful enough to evoke tears in the one who looks upon it, but it will still eventually succumb to death.
In the third stanza, Herbert discusses the beauties of spring more collectively as "sweets," including such things as beautiful days and blooming roses. All of these things, though they may be lovely, have "closes" and will come to an end.
The final stanza, then, turns to the idea of virtue as the only thing which does not follow this same pattern. A "virtuous soul," although it is beautiful, is less akin to the delicate and fragile beauties previously described than it is to "season'd timber." This image suggests a sturdiness and rigor to the virtuous soul, which is lacking in something like a rose or a spring day. Unlike these things, the soul does not yield but "lives" even after the rest of the world has become "coal."