The [ð] sound is one of the two sounds usually represented by the English letter combination th. (The other is the [θ] sound.)
This symbol used to be a letter in the Old English alphabet and still is a letter in the Icelandic alphabet. The name of the symbol is usually pronounced "edh" [ɛð].
If you're having problems hearing the difference between [θ] and [ð], try the discrimination practice.