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 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
Final, after u, it would evolve into w: kuȝ (KUȜ) > kuw
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 *ghaladâ/ȝaladâ in the Quenyan branch.
Once more, the gh sound would disappear again, and in Quenya it
Note that in other languages, like Orkish, Tolkien wrote this sound as gh.