It means "one fifth" (20%). It's like saying "the lower half." After an adjective, we don't say "one half"; we just say "the blue half," "the smaller half," etc. The same is true for other percentages. "The lower third of the painting is damaged" = one third of the painting is damaged, and it's the part of the painting at the bottom. If you drew a scale with all the counties ranked in order according to their wealth, the one-fifth of the scale with all the poorest counties would be the "poorest fifth" of counties.

a fifth = 20% five = five