For a poem about a dead guy, Jonson doesn't spend a ton of time talking about, well, death. What he does instead, however, is talk about stuff that's related to death like tombs, monuments, and other forms of memorialization. According to Jonson, all these earthly forms of commemoration pale in comparison to the monument Shakespeare left to himself, his plays and sonnets.
Line 19: Jonson compares Shakespeare to other great poets from Britain's past, Geoffrey Chaucer, Edmund Spenser, and Francis Beaumont, all of whom were buried in a special corner of Westminster Abbey known as Poet's Corner. By saying he wouldn't bid them move over to make room for Shakespeare, Jonson simultaneously creates an image of Shakespeare as being on a different level than these other authors and makes a jab at the fact that Shakespeare was currently not memorialized in Westminster.
Line 22: Referring to Shakespeare as a monument without a tomb speaks to his importance in British literary history and also references the book in which Jonson's poem appeared, Shakespeare's First Folio. The book, Jonson is saying, is all the monument that Shakespeare needs.
Line 65: Children, in their own way, are another kind of memorial to their parents. Shakespeare had two actual children of his own, but here Jonson is referring to Shakespeare's poems as his children; the personality and legacy of the author lives on in his "issue," or his work.
Line 75: A common convention in memorial poems from Jonson's time period is the idea of turning dead people into constellations. It's something people picked up from the ancient Greeks and Romans and comes from the belief that the stars were eternal (never mind that most of the stars we see in the sky have actually already burned up and exploded).