This phrase refers to wise men who had no recognition or glory for their wisdom or words. They went through life and didn't make much of an impression, they didn't "fork lightning" in their speeches or do anything so stunning as forking lightning with their brilliant words. Forking lightning is just an image, a metaphor that Thomas uses to compare to people whose words have a powerful and stirring impact on others. However, these men simply lived their lives, perhaps disappointed with their impact and importance. Thomas is saying that despite the fact that these men might have regrets about how they lived their lives, and the things that they didn't do, they still "rage, rage against the dying of the light." Even they fight off death with every fiber of their beings, perhaps even more so because they felt that they led ineffectual lives, and want more time to make an impact.
Thomas refers to many different types of men (wise men, good men, wild men, and grave men), all who, despite the type of life they lived or who they are, all rage against coming night, or death. Thomas also wants his father to fight it off, to be strong until the end, and die not quietly, but cursing and blessing him, and raging against the "dying of the light".