the wast land

Eliot studied medieval customs and religion and brought this knowledge to bear when he wrote "The Waste Land." Among the books he researched was "From Ritual and Romance" by Jesse L. Weston, but the most influential source work was "The Golden Bough" by Sir James Gregory Frazier. I highly recommend Frazier's book, still readable
and worthwhile today!
The Holy Grail and The Fisher King occur as motifs in the poem, as well as older Biblical traditions. One of the most important themes is redemption for one crying out in the wilderness, i. e., "The Wasteland."
Modern times (immediately following the First World War)
seemed to be a waste land to all of Europe in general and to artists in particular. The poem -- written in 1922 --may have been a response to this universal angst and spiritual emptiness in the world at that time.
What was truly revolutionary about the poem was its form-"free verse" to be sure, but also experimenting with fragmentary lines. There are several speakers throughout the poem, as well quotes and parts of quotes from Scripture and other literary works.
(The contemporary habit of "sampling" in jazz has a very early literary counterpart!)

This is really a perfunctory summary of the poem. Read it more than once, and read some critical analysis of it.