Yogh (Ȝ ȝ) is a letter of Middle English literature. Note that sometimes it is written with a three (3) when the font is not available.
The Middle English sound Ȝ was derived from Old English g. It was a sound that doesn't exist any more in English language since it is now assimilated with y or isn't pronounced at all.
 Ȝ in Elvish
 Primitive Quendian
Before unvoiced consonants it would evolve into ch: maȝtâ (MAȜ) evolved to Common Eldarin machtâ.
Final, after u, it would evolve into w: kuȝ (KUȜ) > kuw
 Later Ȝ
The sound gh would return once more in Elvish. Like in the case of English, Primitive Quendian g was simplified to gh in Quenya (and Sindarin, in most cases). For example the word galadâ "tree" evolved into *ghaldâ/ȝaldâ in the Quenyan branch.
Once more, the gh sound could be heard in Elvish, and it survived long enough to be written in Sarati before disappearing again. The word alda was still spelt ȝalda with the letter ' representing gh. It was known by tradition among the loremasters
When gh was lost once more, the Sarat ' was eventually used for a carrier. It was inherited as a carrier in the Tengwar and later was replaced by the letter Anna, to indicate an assuming “vanished” gh wherever words begun with a vowel.
This usage tried to explain the relation of words between Noldorin words starting with a vowel where Telerin had g- (cf. Quenya alda vs. Telerin galla), but did not explain all the cases of words without an initial consonant (alca, according to this, should be spelled *hDjaE
that time, although it had never been **galka before).
 Other languages
Note that in other languages, like Orkish, Tolkien wrote this sound as gh.