O mighty-mouth'd inventor of harmonies,
O skill'd to sing of Time or Eternity,
God-gifted organ-voice of England,
Milton, a name to resound for ages;
Whose Titan angels, Gabriel, Abdiel,
Starr'd from Jehovah's gorgeous armouries,
Tower, as the deep-domed empyrean
Rings to the roar of an angel onset-
Me rather all that bowery loneliness,
The brooks of Eden mazily murmuring,
And bloom profuse and cedar arches
Charm, as a wanderer out in ocean,
Where some refulgent sunset of India
Streams o'er a rich ambrosial ocean isle,
And crimson-hued the stately palm-woods
Whisper in odorous heights of even.
We all indulge in hero worship now and then, and famous poets are no exception. Tennyson here heaps adulation upon adulation onto Milton, and not without cause. Milton is one of the true giants of English verse, alongside Chaucer and Shakespeare. Tennyson's praise is not slight, either, as he calls Milton the "inventor of harmonies" who was so skilled that he could "sing of Time and Eternity." His voice was so melodious and powerful, so sublime that Tennyson calls him the "God-gifted organ-voice of England" whose name will (and has!) "resound for ages."
The poem concerns itself with listing the amazing accomplishments of Milton's verse, primarily Paradise Lost. Tennyson's own verse in tribute to these strives for dense, descriptive beauty, similar to Milton's own gifted descriptions. However, it's not the "Titan angels" that most interest Tennyson. Though he glorifies the brilliant attire in which Gabriel and Abdiel are clad ("Starr'd from Jehovah's gorgeous armouries") and the way that they stand tall and speak loud in the heavens ("Tower, as the deep-domed empyrean Rings to the road of an angel onset), these characters are not his main concern. Rather, he is more interested in the utterly bewitching setting that Milton crafted with his Eden.
Tennyson would rather wander lonely though Milton's Eden, that "bowery loneliness" filled with "brooks mazily murmuring" (mazily, I assume, meaning winding like a maze). The plant life also enraptures him, for he says the "bloom profuse and cedar arches charm" him. They charm him in the way that a "wandered out in ocean" may be charmed by the "refulgent (brightly shining) sunset of India" as it "streams o'er a rich ambrosial ocean isle." He paints the most captivating picture of a heavenly isle in an exotic land, illuminated, shining, by a sunset which stains the "stately palm-woods" crimson. The whole place whispers with delight. That is what Tennyson takes from Milton's Eden, a sense of utter awe at heavenly beauty. That his imagination ran so far as to produce this lush, gorgeous poem should be enough to show the depth of his inspiration and admiration.
So as to not leave you confused, reader, the italicized text at the top of the poem, Alcaics, refers to the poem's structure. An alcaic stanza is an ancient Greek lyrical meter, and this poem follows that pattern. I believe Tennyson chose a Classical form to mirror the intent of Milton in writing Paradise Lost. Milton wanted to write an epic for the English language, to elevate English poetry to the level of art and legitimacy enjoyed by the great Greek and Latin epics. Tennyson using an antique form seems fitting.