When in April, with its sweet showers, The drought of March had pierced to the root, And bathed every root in such liquor Of which virtue engendered is the flower.
Basically, it’s setting the scene for the coming story (it’s the introduction to The Canterbury Tales), and talking about in April spring starts to see rain dispelling the drought of March and filling the roots with sweet water.
It goes on to talk about the winds and the birds, where the sun is in the sky (directly overhead), and then to pilgrims starting their pilgrimages, as a sort of “And this is where we begin our story” thing.