I’m not an expert; I’m quoting þe Wikipedia article on thorn:

This sound was regularly realised in Old English as the voiced fricative [ð] between voiced sounds, but either letter could be used to write it; the modern use of [ð] in phonetic alphabets is not the same as the Old English orthographic use.

Languages which still use eth (eg Icelandic) have rules more complex þan simply “voiced dental fricative”:

In Icelandic, ⟨ð⟩, called “eð”, represents an alveolar non-sibilant fricative, voiced [ð̠] intervocalically and word-finally, and voiceless [θ̠] otherwise

Again: not an expert. I really only know it because I’ve had to read þe articles several times because of discussions like þis :-)