There are so many things going on in "The Waste Land" and so many literary references that it is an oversimplification to suggest one grand meaning. But in general, the 'wasteland' suggests that the Modern era (then 1922) was the waste land. The tone of the poem is mournful (with only bits of hope, more toward the end); the period after World War I left many people disillusioned, not just about the current state of the industrialized world, but with the idea of progress. In other words, if progress is real, if the world is getting smarter, more advanced and so on, then how can such a war occur. In the first section, the opening line is "April is the cruelest month." April is a time of renewal; but in this context it is cruel; the idea is that renewal should be occurring but it is not.
After all of this talk of a waste land, the thunder becomes audible, "da" (which may be German for "there" - the thunder being there, audible but in the distance) and then "Datta," "Dayadhvam," and "Damyata." In order, they mean "give," "compassion," and "control." These come from the Brihadaranyaka Upanishad, which is an ancient philosophical text containing Hindu and Buddhist concepts. Among these concepts and statements are three duties (giving, compassion, and control). These are things each individual must sacrifice to the gods, to other people, animals, and so on. It is part of the lessons of ethical responsibility for each person.
So, when the thunder "says" these three things (giving, compassion, and control), the thunder is far away - and still no rain. While the landscape is still dry and dead (waiting for rain), the thunder at least offers the possibility of rain (the hope that these things will rain down on the waste land and provide the spring that April has not yet provided). The thunder's potential promise of rain - leading to growth and life - is a parallel to the promise of individual and social improvement. So, the poem ends, not with the achievement of peace, but with the potential of it. The thunder is the hope/potential; the rain, if it eventually comes, is the achievement.