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
Note that in other languages, like Orkish, Tolkien wrote this sound as gh.