Rabbi Yehuda discusses the power and influence of the Torah for the dead. When the soul of one who labors in the Torah departs this world, it ascends by the Torah's familiar ways, and the Torah preserves the body and guards it against the Judgments of the other world until the day of resurrection. However, when the soul of one who does not labor in the Torah leaves this world, it does not know the paths to follow. Therefore, it "stumbles" and receives punishment.
This section also provides an explanation of Reuben's questionable actions, as related in the title verse. The discussion of this issue reveals that Reuben did not actually lie with Bilhah; rather, he disarranged the couch in order to prevent the Shechinah from performing her conjugal duty with Jacob. Thus, Reuben was punished. He was deprived of his birthright, which was transferred to Joseph, in accordance with God's Wisdom. However, Reuben's merits remained intact and his descendants remained worthy of inclusion among the twelve tribes. Similarly, the actions of Eli's son - recounted in the verse, "Now Eli was very old ..." - do not mean that he lay with the women at the entrance to the Temple. Instead, he detained them, preventing them from entering until the other sacrifices had been offered, as was appropriate.
The quality of life we create for ourselves in the physical realm mirrors the quality of life awaiting us after our departure from this existence. Our quality of life is determined by our actions and their degree of spiritual development via the path of Torah. This section enlightens us to the power of Torah and the path it offers. The radiating Light helps us clearly see the darkened corridors of this life, in order to avoid traveling darkened byways in the hereafter.