Enotes provides great summaries on a variety of different poems, stories and books. Below, I have provided a link to a very in-depth summary of the poem, and that should be useful. But in a nutshell, the poem is about how all of us are always waiting in anticipation for wonderful things to happen to us. We waste our lives away just sitting around, waiting for these moments to drop into our laps. He compares the moments to a beautiful ship that we see sailing towards us, and we think that ship will "heave to and unload/All good into our lives, all we are owed/For waiting so devoutly and so long." But, he says that we are inevitably disappointed by each of these anticipated moments. When each one comes, it is "No sooner present than it turns to past." They are so short in time, passing so quickly. Sometimes, they pass us by completely. But, the only thing that we can count on for sure is death; it will always come, it will never disappoint, and will last forever.