- the "y" sound, as in our "yes"
- fricative phonemes rather like the "ch" in "loch"
The letter came to be represented in print by a figure quite similar to a "3". In the passage below (also from the Tolkien/Gordon edition of Gawain), you can see a yogh in each of the top three lines. Many other editions of the poem (including the one I met in school) have had the various yoghs replaced by letters which more clearly mark their pronunciations for a modern audience. In R.A. Waldron's edition, for example, the three "3's" below have been replaced by "gh's" indicating that they were meant to be pronounced as fricatives.
The letter lasted longer in Scots than it did in English, but even there it began to cause some confusion, a small effect of which I'll post tomorrow.