The poem begins with the speaker coming across a strange sight. First, there's a spider. Nothing unusual there, except that this spider is "dimpled" (meaning it has little indentations in its body) and fat. Such adjectives are usually used when talking about human beings, and their use here might make this spider seem a little cartoonish.
The consonance of "found a dimpled spider"—with all those bloated /d/ sounds in particular—seems to evoke the spider's rotund appearance. Notice, too, how that caesura between "spider" and "fat," makes the line feel almost like it's swaying, as though the spider is nearly too heavy for the flower it sits on.
This spider is also "white," a relatively unusual color for such a creature. This big, white spider doesn't seem all that odd, however, until readers get to the next two lines: the spider is resting on a "white heal-all" (a small flower, which is normally blue), and it's holding onto a white (presumably dead) moth—it's next meal!
The spider might now seem grotesque and gluttonous rather than merely odd. And the fact that there are three white creatures together makes the scene all the more surreal. The speaker repeats the word "white" three times in three lines, in fact, emphasizing just how wild of a coincidence this is (note that this kind of repetition is called diacope).
The white moth, meanwhile, is stiff with death, like a "piece of rigid satin cloth." This simile makes it seem as though the spider is inspecting the moth for quality, admiring its deadly handiwork while remaining callously indifferent to the life it has just taken.
The flower's name thus becomes rather ironic: heal-alls have long been utilized for their supposed medicinal properties, and their name itself suggests positivity, good health, and the wonder of nature. The color white also often symbolizes purity, holiness, innocence, and so forth. There's a dawning tension, then, between the connotations this white scene might evoke and what is actually being described.
Finally, note the meter of these lines: Frost is using something called iambic pentameter, which means that there are five iambs per line. An iamb is a poetic foot that follows an unstressed-stressed syllable pattern. Here's the meter of line 1 as an example:
I found | a dimp- | led spi- | der, fat | and white,
This bouncy da-DUM pattern makes the lines feel bouncy and musical. The meter, like the whiteness of the scene, is perhaps deceptively simple and upbeat.