Fert stands twenty second, writes f, and carries the numeral 500 from Greek phi. Its name is the one item in the alphabet's vocabulary that nobody has satisfactorily explained.
The sound is the point. Slavonic had no native f. Every word in which the letter occurs is a borrowing, almost all of them from Greek and most of them proper names or terms of the church, so the letter exists in the alphabet to write words that came in with the religion the alphabet was made to serve. That is a common pattern in the history of writing and it is unusually clean here, because the borrowing and the alphabet arrived together.
Speakers regularly substituted sounds their language did have, and the manuscripts show it: names spelled with this letter in one copy appear with p or with a cluster in another. Those substitutions are useful evidence, because they show how the Greek vocabulary was actually being pronounced by Slavs rather than how it was supposed to be.
The letter has a companion, fita, which also writes f and which exists to transliterate a different Greek letter. Which of the two is written in a given word depends on the Greek spelling behind it, and knowing that is a matter of vocabulary rather than of sound. Both survived until the modern reforms; Russian abolished fita in 1918 and kept fert.