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.
The Waste Land is a tricky compilation of Eliot's meanderings and responses to current events, mythology, and obscure texts.
Fortunately for the reader, Eliot took it upon himself to make a series of notes at the end of the poem to explain some of his references.
In the section of "The Waste Land" entitled 'What the Thunder Said,' Eliot uses personification, making the rumbles of the thunder into a conversation piece. This is actually based on a Hindu fable about what thunder says when it rumbles “Datta, dayadhvam, damyata” (Give, sympathise, control)" (from Eliot's note for line 401).
Representing the coming of a destructive storm, Eliot's 'What the Thunder Said' is the last section of the poem.
These three terms are adopted from UPANISHADS of INDIA'S ancient holy scripture. when thunder appears following the rain fall the gods, men and evil spirits demand an answer for the moral existence of human beings. the three different paths " to give ,to sympathize, to control ordered by the thunder should be the manthra for people on earth. they must abstain from the war ,help others and to have a control on their evil thoughts.km
What the thunder says is "Da" which is interpreted by three parties differently. It is interpreted as "Datta" by men which means "give" as men are naturally avaricious. The demons interpret the sound as "Dayadhavam" which means "compassion" as they are cruel. Finally, the Gods interpret it as "control" as they are uncontrollable. It is expected that by listening to what the thunder says, the waste land can be nourished and repaired.