"On Shakespeare, 1630" was published at the start of a collection of Shakespeare's plays known as the Second Folio. As such, it's no surprise it sets out immediately to sing Shakespeare's praises! Just like a modern-day collection of poetry might feature gushing words from fellow poets on its book jacket, this poem hypes up its subject in order to get readers excited and engaged. Milton was pretty young at the time of writing, so readers might think of this as him doffing his hat to one of his major literary influences.
The poem itself opens with a rhetorical question: why should someone as singularly great as Shakespeare need a stone monument built to commemorate him (to house his "honoured bones")? There is no need, the speaker implies, to toil and sweat to make a physical tribute. A pile of stones seems kind of pathetic when weighed against the sheer longevity and majesty of Shakespeare's total output.
Note, too, how the speaker says "my Shakespeare." The word "my" suggests familiarity and intimacy; rather than treating Shakespeare like some distant, lofty literary figure, the speaker suggests that Shakespeare belongs to all his readers. (As the speaker will go on to argue, Shakespeare doesn't need a monument because he lives in the hearts and minds of his readers and audience members. We, the people, are his living monument.)
Lines 3 and 4 reiterate the point made in the first two. Why should Shakespeare's holy bones ("hallowed relics") be hidden away under a "star-ypointing pyramid" (by which the speaker means a pyramid-shaped tomb pointing towards the sky; it's also possible that Milton here is riffing on an epitaph for Sir Edward Stanley that Shakespeare may have written himself, which mentions "sky-aspiring Piramides"). The broader point is that however impressive and expensive, no stone monument is sufficient to commemorate Shakespeare's genius.
"On Shakespeare 1630" is written in iambic pentameter, the classic Shakespearean meter. This means each line contains five iambs, poetic that follow an unstressed-stressed syllabic pattern (da-DUM). The poem is also written in heroic couplets, meaning the iambic pentameter fall into in rhymed pairs. Here are the first two lines to show this pattern at work (take note that "pilèd" is two syllables, not one):
What needs | my Shake-| speare for | his hon-| oured bones,
The lab- | or of | an age | in pi- | lèd stones,
The steady meter and quick, full rhymes make the first sound confident and forceful.