Edh (or eth) is a letter in the Anglo-Saxon and Icelandic alphabets.
ð in lowercase and Ð in uppercase.
It has the voiced th sound as in "them".
In fact "y" is often substituted for it on gravestones and quaint store signs: "ye olde candies shoppe" should be read as "THe olde....", although it is jocularly pronounced "yee".