The sonnet “On Shakespeare” is a tribute from Milton to Shakespeare.
The first four lines are saying that Shakespeare does not need a pyramid ("Star-ypointing pyramid") that takes ages to build in order to honor his life. Remember that a pyramid is a tomb that took a very long time ("the labour of an age") to build.
The next four lines state that building Shakespeare a monument would be pointless. He does not need a monument (“what need’st thou such weak witness of thy name?”) because he has already built the monument for himself.
What is the monument he has built? The next four lines are an important clue:
Milton says that while other poets are slow (“the shame of slow-endeavouring art”), Shakespeare’s poetry flows easily (“thy easie numbers flow”). He also goes on to say that the reader takes from what Shakespeare has written a meaning that may come from the gods. (Delphick is an allusion to Delphi, where Apollo, the Greek God of poetry, visited the oracle). His monument, therefore, is his work.
The poem ends by telling us that Shakespeare is in a tomb a king would envy:
The reader him or herself is the tomb. When we read Shakespeare’s words (“our fancy of it self bereaving”) we are amazed (“dost make us Marble”); we become the material of his tomb (marble). He does not need a pyramid because his words produce everlasting memory. Kings may have pyramids, but they don’t have that.