In this poem, Bronte uses rain to serve as a metaphor for gloom, despair, and even death. Early in the poem, she remarks that "morning rain" can lead to a "pleasant day." She adds that since rain makes the flowers bloom, we should not be gloomy or sad when the rain does fall. As the poem continues, Bronte uses the metaphor to address human concerns. At times, the rain will fall. Likewise, there will be times when humans must deal with Death. There will be times when "sorrow seems to win, / O'er hope." However, just as sunny days and blooming flowers follow the rain, hope and happiness will follow such sorrowful times.
Note that spring is the season of rebirth. Bronte adds that "Hope again elastic springs." This means that hope is elastic; it can bend but will not break. Hope will spring back into action, just as the spring season will always return after a cold winter.
The idea of rain sets up the metaphor. The rain becomes a vehicle for describing gloomy weather. Bronte uses this vehicle to refer to human sorrow. The upside is that just as sun and life follow the rainfall, hope may also "spring" back after human sorrow. In this way, the rain (or its referent sorrow) is not something to dread. It is simply part of the ebb and flow of life and emotion: rain/sun, sorrow/hope.