In the poem, Milton says that the human being is eternal. He triumphs over time by attaining eternal life in heaven. Time is personified as being envious by human beings. Poet asks time to flee as it will soon face defeat and humans will face eternal bliss.
The “lazy leaden steeping hours” refers to clock moving slowly covering hours with “leaden” hands. Times movement is compared to the pendulum in a clock that moves slowly. Times movement is limited to the clock but human life is eternal and exists even after death.