The speaker of the poem (Thomas) implores the reader to fight and resist death even if it is inevitable. "Wise men" and philosophers will rage against death because their wise words may not have had the effect they had wished. Good men rage against the "dying of the light" (death) because even though they had been good, their lives were not as exciting as they could have been. Wild men rage because they had lived full lives, had not worried about death, and were therefore not prepared or ready to die. Blind men see death perhaps more acutely than those with sight. They have some experience with the darkness associated with death. Thomas's father was blind and this is significant here. 

In this last stanza, Thomas is addressing his father particularly. He notes that his father is on a "sad height" - on the precipice of death. He also wants his father to rage against death. This is not necessarily because of regret or being unprepared. He simply wants his father to fight. So, he wants his father to curse (fight) and bless him (making him feel better) with fierce tears. In crying fierce tears, he shows that he is fighting (fierce) against his own sorrow (tears). Thomas will feel blessed that his father is also raging against the dying of the light. To rage against death is to show one's love for life and reluctance to leave his loved ones.