Fritu writes the consonant f and takes the numeral five hundred. It exists because the alphabet had to be able to spell Greek, not because Slavonic needed it: the sound f was not native to Slavonic, and the words in which the letter appears in the earliest texts are almost entirely names and terms taken from Greek along with the scriptures and the liturgy.
That makes the letter a marker. Where it appears in a Slavonic text, a borrowed word is usually being written, and the letter is one of the plainest signs in the alphabet of the fact that the script was made for a translation project. A script devised for the everyday speech of a Slavic people would not have needed it.
Its name is the first in the alphabet that cannot be read as an ordinary Slavonic word. The reference literature records fritu, and the corresponding Cyrillic name fert, as of uncertain or unknown origin, and no etymology has been established for either. The atlas records that as an open question rather than proposing a reading.
The numeral five hundred follows from counting the Glagolitic letters in their own order, this being the twenty third sign, and it agrees with the value Cyrillic inherits from Greek phi.