Zing is a quality of excitement or enthusiasm. If you want to add some zing to your nephew's birthday party, you might want to hire a magician or offer pony rides.
A director might add zing to her movie by enticing a big movie star to play a role, and your brother may tend to put extra zing in his stories in the form of exaggerated, colorful details. In both examples, zing makes something that's slightly dull seem a bit livelier. You can also use zing for a spicy or piquant flavor: "I put some cayenne pepper in the sauce to give it some zing."