To understand the importance of this symbol you might like to think about the various connotations that we have with rain and how it relates to growth, rebirth and nurturing.
In the same way, this novel makes it clear that rain is in a sense the lifeblood of the community, as they are dependent on rain each year to fill the river and water their crops. As the novel progresses, we see that much of their ritual celebrations are focused around their relationship with rain, as it has the power to change the community from one that is productive and has an abundance of food to one that is starving.
What is key in this chapter is the way in which Waiyaki regards the rain as both a blessing that brings life and yet at the same time as a curse which snatches life away because it takes the soil from the earth, making the land unable to produce good crops. His thoughts about the rain parallel his own internal conflict as he wavers between trying to reconcile his desire to achieve progress in his tribe and plan for a future but also at the same time retain valuable cultural links with the past.