The answer to this question can be found in the first stanza, which contains two of the lines that are repeated so often in this excellent example of a villanelle, which involves repeating certain lines in a set format throughout the poem. Note what the first stanza says:
Do not go gentle into that good night,
Old age should burn and rave at close of day;
Rage, rage against the dying of the light.
Thus we can see tha this first stanza outlines two possible responses to death. Firstly, you can "go gentle into that good night," accepting the inevitability of death and not fighting against it, rather choosing to die quietly and to fade away without a struggle. Secondly, the alternative option is to "Rage, rage against the dying of the light," or to meet death with a grim struggle, that will not give in without a fight or a conflict. Thus the two ways of meeting death are polar opposites in terms of the passive and active way in which they suggest we should meet death, and the emotions of peace and calm acceptance on the one hand, and rage and anger on the other.