The poem begins with a disorientating simile that contains two propositions. The first line describes grief as something imperceptible—a statement that the poem will go on to reinforce. Grief can be many things, but it's not often associated with being something difficult to perceive. Grief, of course, can sometimes be loud and expressive—think of mourners crying at a funeral—or more inward and quiet, but in both of these situations it remains perceivable, to the person who feels it at least. The poem is thus putting forward the idea that there is a different kind of grief, one that functions more subtly than the reader might expect. This grief is almost beyond the realm of human perception—but the poem shows right away that it wants to subtly point the reader in its general direction.
Moreover, the opening line hints at the way time and change may or may not be perceptible. Think of a slug or a snail: it can't perceive fast movements because its sight only refreshes at a certain speed (a bit like a camera's frame rate). Lurking in the atmosphere of this poem is the idea that there are things that humans are similarly unable to perceive. The first line is an oxymoron: grief is a human emotion, and yet the line claims that there is a kind of grief that is almost beyond human powers of perception. From here on out, it's up to the reader to engage with the poem to figure out just what loss this kind of grief might mark.
The second proposition comes in line 2, which completes the simile begun by the poem's first word. The speaker says that the dimly-perceived experience of grief is similar to the way in which summer disappears—and, in this poem's setting, has already disappeared. In the poem's present moment, the summer has moved from being a daily reality to a memory, a kind of gentle loss. "Lapsed" is an especially interesting choice of word. Its most obvious meaning here is that of "passing gradually," but it also has a subtle suggestion of something being lost. For example, "lapse" is often used in the context of forgetfulness, as in a lapse of memory—perhaps, then, this word hints that people might forget to perceive something fundamental about life. The poem subtly draws the reader's attention to the way that the relentless passage of time imbues everything with a sense of loss.
Lines 3 and 4 expand on the discussion of the faded summer. In essence, they argue that it would be a mistake to accuse the summer of being deceptive or dishonest ("perfidy"), because the shift in seasons happened so slowly. The near-repetition of "imperceptibly" neatly embodies the idea of subtle, gradual change—"imperceptible" is the same word without the last syllable. Behind this idea is an acceptance that nothing can last forever—the summer, with its positive associations of sun, warmth, and growth, comes to an end.
Line 4 concludes with a firm end-stop, which sets up a pattern in which each subsequent quatrain does the same. Perhaps this use of the full stop is a subtle way of making the form represent seasonality, with the number of stanzas corresponding to the different seasons (though not discussing them specifically).