I think we can answer this question by first understanding what the title The Waste Land means. Eliot appears to view the present as a post-historical world, as if achievement and progress are over and nothing remains to us except a kind of wreckage, symbolized by (among other things) imagery from mythology, quotations from literature, and allusions to scraps of information from the collective consciousness. For example, he alludes to the Greek myth of Philomela, the woman who was transformed into a nightingale after being raped by her brother-in-law Tereus:
The change of Philomel, by the barbarous king
So rudely forced. Yet there the nightingale
Filled all the desert with inviolable voice,
And still she cried, and still the world pursues . . .
Eliot is following the literary tradition of the nightingale as a symbol of sorrow and pain. You might wish to compare and contrast his allusion to the bird with that of earlier poets such as Keats and Matthew Arnold.
In The Waste Land Eliot's use of myth is multi-layered in the sense that he alludes to modern transformations of myth as well as the original stories. He gives a quotation from Wagner's Tristan and Isolde:
Frisch weht der Wind
Der Heimat zu,
Mein Irisch Kind,
Wo weilest du?
In the story of Tristan and Isolde (or Iseult) the Irish princess is taken from her homeland of Ireland to marry King Mark of Cornwall. Probably, Eliot invokes this legend because of its theme of alienation. In The Waste Land humanity in general is shown as alienated, disconnected from the modern world. We know at least that Eliot himself and other intellectuals felt that way.
The literature of the past, for Eliot, is itself a kind of mythic framework to which he alludes repeatedly in his depiction of the decline of civilization. The very opening of the poem, "April is the cruelest month," is a metamorphosis of the prologue from The Canterbury Tales; Chaucer's famous lines about the renewal of spring become a pessimistic symbol of the darkness of Eliot's time. Dante's lines about Hell are applied by Eliot to modern London:
A crowd flowed over London bridge, so many
I had not thought death had undone so many.
Eliot's long poem is itself an analogue to the world he describes. It is a kind of collection of fragments, of references to the mythic past and to the present, and quotes from literature and popular culture (such as a dance called "The Shakespearean Rag"). All of it depicts a shattered world in crisis which, to Eliot, is like a wreckage filled with the waste of the past.