In my opinion, the meaning of this poem is that life goes on at all times and in all circumstances and that there is some aspect to life that is always beautiful. The poem is actually literally talking about nature, but I think that you can see it as applying to human life as well.
In the poem, beauty can be found no matter whether it is too hot for birds (things that we usually think of as beautiful) or too cold for just about any life. In both cases, things seem bad, but there are still these things (the cricket and the grasshopper) that make life beautiful.
By analogy, you can say that there are times in our lives that may seem bleak, but if we look hard enough, we can find things that are beautiful and should give us hope.