In this case, the ي in the middle of جيد is actually acting as the consonant 'y' (both ي and و do double duty as vowels and consonants, kind of like the 'i' and 'v' of Classical Latin). The double sign (called 'shaddah' or 'tashdiid') is showing that here, this 'y' sound is long, thus you get "jayyid"