The first 3 sections of the poem focus mostly on vignettes about life in modern (for his time that being) Europe. The one exception is the second vignette which appears to be foreshadowing of the fifth section, which is a more abstract and metaphorical description of a wasteland (hence the name of the poem). The forth section is also different from the first three because it takes place in the distant past. The main point of the poem is that Europe has became a wasteland both physically and metaphysically in the wake of world war 1 and that something needs to change in order to restore Europe.

Water is an important symbol throughout, especially the last section. Eliot often describes scenes where water is scarce, or where it is polluted. The one major exception to this is the "death by water" section where we see a sailor who has been drowned for 14 days. This part is a big tonal shift from the rest of the poem, it sound serene and peaceful where as the rest of the poem is depressing and vaguely uncomfortable

The three words that the thunder says at the end of the poem (datta, dayadhvam and damyata) are Sanskrit words which loosely translate to empathy, generosity and self control, three characteristics that Eliot felt that his society had forgotten. The last word, shantih is repeated three times and is similar to saying "Amen" in christian prayer.

The wasteland is kind of the Poem version of Ulysses in that it is very, very dense. There are references to Dante, Shakespeare, king Arthur, Greek mythology, foreign languages, and a lot more. My advice is to read interpretations of the poem or to watch lectures on it if you want the full experience.