The poem begins with the speaker directly addressing spring's "Fair" (or beautiful) "Daffodils," an example of apostrophe:
Fair Daffodils, we weep to see
You haste away so soon;
The speaker laments the fact that the flowers wilt so quickly. In referring to themselves as part of a collective "we" rather than as an individual ("I weep"), the speaker conveys that they're talking about a general human experience. That is, the speaker's "weep[ing]" over the dying daffodils seems to represent some deeper, more collective grief.
The speaker then says that the "early-rising sun / Has not [yet] attain'd his noon." In other words, the speaker is upset about the daffodils dying before the sun has even reached its zenith in the sky. This reflects the fact that daffodils are some of the earliest flowers to blossom in spring and also some of the first to depart. In "hasting away" before the sun has reached its full height, they're missing out on the brightest part of the day.
The personification of the "sun" in line 3 suggests that the speaker's sorrow about the daffodils' "early[]" death really reflects their own anxiety about not reaching their "noon," or peak. The speaker, perhaps, fears not living up to their own potential or missing out on something great; they worry they will die before they achieve whatever it is they hope to achieve in life.
The first four lines of the poem are written in common meter. This means that lines alternate between iambic tetrameter (lines of four iambs) and iambic trimeter (lines of three iambs). An iamb is a foot made up of an unstressed syllable followed by a stressed syllable, and this pattern results in a pleasing and predictable rhythm. Scanned, the first two lines look like this:
Fair Daf- | fodils, | we weep | to see
You haste | away | so soon;
Note that the first foot is not iambic, or at least not strongly so. We'd argue it reads most naturally as a spondee (two stressed syllables in a row), drawing attention to the speaker's beautiful but fleeting subject. Otherwise, the iambic rhythm is familiar and musical. These lines use the ABCB rhyme scheme typically associated with common meter as well (meaning that the even-numbered lines end in a rhyme—in this case, "soon" and "noon").
These opening lines also contain /wee/ alliteration and /ee/ and /ay/ assonance ("we weep to see," "haste away," "attain'd"), /f/ consonance ("Fair Daffodils"), and /s/ alliteration/sibilance ("so soon," "see," "haste," etc.). All of this strong musicality right off the bat suggests the intensity of the speaker's feelings.