Lines 1-2 of "Dreams" consist of an imperative (instruction or command) followed by the beginning of an explanation. In other words, the poem's speaker is offering and justifying a piece of advice.
Who is the speaker, and whom are they addressing? The speaker's authoritative tone suggests that their experience has provided some insight on the subject of dreams. The lack of other identifying context (either in the title or the poem) suggests that the speaker is more or less equivalent to the poet and that their advice is addressed to readers in general.
What kind of "dreams" does line 1 refer to? The word could literally mean visions experienced during sleep. More broadly, it could mean hopes, aspirations, imaginative escapes from reality (as in daydreams), artistic visions, fantasies, illusions, or some combination of these. The context points toward the second category of dreams, since it makes more sense to remain attached to meaningful hopes than to semi-random nocturnal visions!
The word "fast" (line 1) contains a potential double meaning. In context, it primarily means tightly or securely. "Hold fast" is another way of saying, "Hold on tight." But another definition of "fast"—quickly—may be relevant, too. Line 2 indicates that dreams can "die," so the speaker may be warning the reader to grab hold of dreams both securely and quickly, before they slip away (or before they're taken away by some outside force). How this advice translates to real life is open to interpretation, but the general sense is clear: the speaker wants readers to take their dreams as seriously as possible, as soon as possible.
The sound of these lines underscores their urgency. They're terse and full of punchy monosyllabic words. The poem's basic meter is iambic dimeter (meaning each line has two iambs, poetic feet with an unstressed-stressed beat pattern). As such, lines 1-2 contain at least four strong stresses:
Hold fast to dreams
For if dreams die
However, Hughes often employed metrical substitutions and variations. (As a poet who helped incorporate jazz techniques into modern poetry, he liked to keep his verse rhythms a little unpredictable.) Lines 1-2 could also be scanned as follows:
Hold fast to dreams
For if dreams die
In other words, Hughes may want the reader to hear spondees (stressed syllable + stressed syllable) rather than iambs (unstressed + stressed) at the beginning of the first line and the end of the second.
Regardless of the exact pattern, these strongly stressed monosyllables make the lines sound emphatic. Alliteration reinforces the emphasis as well: "dreams"/"dreams"/"die."