Anything that sounds like jazz music, or in other ways reminds you of jazz, is jazzy. The spirited, toe-tapping version of "Happy Birthday" you sing to your grandpa might be described as jazzy.
There are jazzy covers of staid songs, or jazzy ensembles that bring some elements of jazz to whatever they're playing. You can also use this adjective to describe something that's fancy in a flashy or ostentatious way: "She wore a long, jazzy gown that was covered in glittering sequins." Jazzy, coined around 1919, comes from jazz, which probably stems from the Creole slang jass, "strenuous activity."