Voiced is like the th in the, unvoiced is like the th in thin.
Unvoiced sounds the same whispered, whereas voiced loses its buzz when you whisper.
Voiced (eth):
this that then with the then breathe bother those though
Unvoiced (thorn):
thread thin thanks width breath both youth pithy smith thatch thought throughout thorough
Interestingly, Icelandic and Old English used thorn for the voiced one too, but with the introduction of eth, that usage dropped out of Icelandic and with the introduction of the printing press, y got substituted for thorn in English, resulting in "ye" for the, which was never pronounced "yee", always "the".
Arguably, the printing press came at exactly the wrong time for English, which was at a time of language change and inconsistency, and we got stuck with some very inconsistent spelling. For example, the letter cluster (grapheme) ough represents different sounds (phonemes) in though (oh) thought (or) throughout (oo, ow) thorough (uh), partly because the Old English/Lowland Scots sound gh was on its way out. In Lowland Scots (much closer to Old English than Modern English), night is pronounced similarly to the German word nicht, but gh is voiced when ch is unvoiced.