Heru writes a voiceless velar fricative, the sound conventionally transliterated kh, and takes the numeral six hundred. Unlike the letter for f, this one records a sound Slavonic genuinely had, and the letter is used in native words as well as in borrowings.
Its name is opaque. The reference literature records it, and the corresponding Cyrillic name kher, without an etymology, and it belongs to the run of names at the end of the alphabet whose sense cannot be recovered. That is the second of the two things the letter is chiefly cited for.
The first is its shape, or rather its shapes. Glagolitic has a second sign for a sound close to this one, encoded in the Unicode Standard under the name spidery ha, and that second sign belongs to the Croatian branch of the tradition rather than to the common inventory of the earliest manuscripts. Two signs for one sound, one of them regional, is exactly the situation that makes the total number of Glagolitic letters a matter of how you count.
The numeral six hundred follows from counting the Glagolitic letters in order, this being the twenty fourth sign, and it agrees with the value Cyrillic inherits from Greek chi.
The sound itself is one of the more useful ones in the alphabet for a comparative reader. Where Slavonic writes it in a native word, the modern Slavic languages generally still have it, and where it stands in a borrowed word it usually renders a Greek chi, so the letter marks the seam between the inherited vocabulary and the imported one. Reading a page of Old Church Slavonic with that in mind gives a rough sense of how much of the text came in with the translation and how much was already there.