In this poem, Dylan Thomas is at the deathbed of his father, and is pleading with him to die with honor and pride, fighting until the very end. He doesn't want his father to just give up and turn his life over, he wants his dad to fight death off, to "rage, rage against the dying of the light" and to stay with him as long as he can. Throughout the poem, Thomas gives examples of all types of men who, when dying, fight it off. His gist is that it doesn't matter what type of life you have led, or what type of man you were, all men should fight death off.
In the second stanza, Dylan Thomas refers to how "wise men" know that dying is the right thing to do; they "know dark is right." So, before death, they know it is their time. But, still, these wise men regret leaving the earth because "their words forked no lightning." This is just a fancy way of saying that they never had much influence with their lives; they never gave great speeches with powerful words that inspired people, they never wrote profound novels or essays, and they never spoke or inspired with their words in a way to make them be super influential in life. "Forked no lightning" is a way of saying their words didn't have power, intensity, and beauty (like lightning). And, because these men's words did not do this in their lives, they don't want to die, and so they too "do not go gentle into that good night" when they die. They regret not having been more profound while alive. That is just one of the type of men that Thomas refers to in his poem, of the many that "rage against the dying of the light."
I hope that those thoughts helped; good luck!