It's the icelandic and old english letter for the sound at the beginning of "thorn", not to be confused with eth (I don't know how to display it, but it looks like a partial derivative with a slash through it) which is for the sound at the beginning of "the".
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One is the airy th sound of 'thorn' or 'earth' or 'thick '
and the other is vocalized, as in 'the' or 'then.'
I'm sorry I don't remember the proper linguistic terms.