Lines 1-4 describe the poem's setting: a calm, beautiful evening, complete with a lovely sunset. (As lines 5-8 will reveal, the speaker is observing this sunset on the coast and might be walking along a beach.) The description emphasizes the speaker's feelings of peace and reverence:
It is a beauteous evening, calm and free,
The holy time is quiet as a Nun
Breathless with adoration; the broad sun
Is sinking down in its tranquility;
"Beauteous" is a synonym for "beautiful"—a grand, old-fashioned word that captures some of the grandeur of the scene itself. "The holy time," meanwhile, refers to the time of evening worship in some Christian denominations (i.e., around sunset).
A simile in lines 2-3 then compares the "quiet" of this hour to "a Nun / Breathless with adoration." In other words, the atmosphere is so windless and peaceful, it's like a religious woman whose worshipful love has taken her breath away.
This personification of the "holy time" can be read as a projection of the speaker's own feelings; after all, it's the speaker who's entranced by the beauty of the evening. The setting sun looks "broad" and "tranquil[]" as it "sink[s]" in the sky, again reflecting the calm, expansive mood of the speaker.
These are the first four lines of a sonnet, and they follow the ABBA rhyme pattern of an Italian/Petrarchan sonnet (whose first eight lines conventionally rhyme ABBAABBA; note that the next four lines will depart from this scheme slightly).
Like all traditional sonnets, this one has 14 lines and is written in iambic pentameter (meaning each line has five iambs, poetic feet with an unstressed-stressed syllable pattern). Its rhythmic variations on this meter often have an expressive effect: for example, iambic lines start with an unstressed syllable, but line 3 actualy begins with a stressed syllable: "Breathless." Combined with the enjambment over the line break ("a Nun / Breathless with adoration"), this effect places extra emphasis on "Breathless," making the word sound like an impassioned release of emotion.