To understand the message of this poem, it is helpful to know a bit about the context. Teasdale wrote this poem in 1918, as World War I was coming to an end. Teasdale was inspired by the unprecedented levels of violence and destruction that she, and the rest of the world, had witnessed during this war.
In the world of Teasdale's poem, mankind has been wiped out as a result of its thirst for violence. The only survivors belong to the natural world. The trees, the birds, and the frogs continue on, not knowing or caring where mankind has gone.
"There Will Come Soft Rains," then, highlights the pointless and futile nature of war. The death of mankind changes nothing in the world. People are gone, but the natural world continues unaffected. This message also functions as a warning to the reader. Teasdale argues that war is pointless and that if we continue fighting on the kind of level seen in World War I, we are certain to achieve nothing but our own destruction.