In the text "Do Not Go Gentle into That Good Night" what do the "wise" men realize before they die? How is death presented in this poem?

According to Dylan Thomas' poem "Do Not Go Gentle into That Good Night," wise men, when they reach the end of their lives, "know dark is right." This suggests that they realize that the time has come for them to die--symbolized by "dark." Since they are wise, they know they cannot live forever, but they wish that their words would have "forked ... lightning" during their years on this globe--that is, that they would have been able to, through their wisdom, say things that could have caused positive change among those they are leaving behind. Because they will no longer be able to influence the world, they do not give up their lives easily. 

The repeated lines in the poem present death in two different ways. First, it is "that good night." Dissecting this phrase, we can take the word "that" to indicate it is a well-known commodity; it is something everyone has always had to face, and something that everyone must face sooner or later. It is "good." This seems ironic considering the poet urges his father to not succumb passively to death. Perhaps the "night" of death is good, that is, acceptable, once one gets there, but that doesn't mean one should be in a hurry to leave this life behind. "Night" can have either positive or negative connotations. While night can be a time of fear and isolation, many people enjoy the night because it is a time for sleep and rest, a time to enter into a different dimension in a way--the dimension of sleep. Taking the words "good night" as a unit, they remind us of parting words of blessing before one goes off to sleep. Thus "good night" has an almost soothing feel to it even as it contains ironic and foreboding tones.

The second repeated line describes death as "the dying of the light." This phrase is much less amenable to a positive connotation. Light is universally considered preferable to darkness; it suggests not only understanding, but also joy. One who is alive has an undeniable "light" in his eyes that connects him to others; once that light goes out, the connection with those on this earth is lost. 

Describing death as "that good night" and "the dying of the light" makes death not necessarily fearsome, but definitely poignant and worth resisting until the last possible moment.