fifth: the one that comes fifth in a counted sequence, and by extension a fifth part (one-fifth) of something
This is the ordinal number built on the word for "five." Its main job is to mark position in a series: the fifth day, the fifth month, the fifth year, the fifth son, the fifth in a list of officials or lots. Wherever people or things are being counted off in order, this is the word that lands on number five.
From that ordinal use comes a second, closely related sense. When something is divided into five equal parts, "the fifth" becomes the name of one of those parts, a fifth of the whole. This is how the word works in Israel's restitution laws, where a wrongdoer must repay what he owes and add a fifth on top, and in the account of Joseph giving Pharaoh a fifth of Egypt's harvest.
The two senses are one idea seen from two angles: counting off to the fifth spot, or slicing something into fifths and naming one slice. English keeps them together too, since "a fifth" can mean both the fifth item and one part in five.
One unusual case is the description of the sanctuary doorposts in 1 Kings, where the word seems to give a proportion rather than a position, and translators disagree about exactly what proportion is meant.