Good men, the last wave by, crying how bright Their frail deeds might have danced in a green bay, (lines 7-8)
Most of the things in this poem are transient, passing easily way, including lives. Each generation rises up like a wave and then, the speaker implies, they crash on the shore and are gone. Not only do their deeds pass away, but also some of the deeds don't even have a chance to happen in the first place!
Wild men who caught and sang the sun in flight, And learn, too late, they grieved it on its way, (lines 10-11)
That's the world for you – life flies by, and as quickly as we can get excited about something, it's gone.