Something frothy is composed of many small air bubbles. You might like your cappuccino frothy but probably not your tap water.
If you order a cappuccino at a cafe, you'll get a cup of strong coffee topped with frothy milk, and a cake recipe might instruct you to beat four eggs until they're frothy. In both examples, air bubbles cause a liquid to become foamy and creamy. A frothy person, however, is bubbly in a different way, full of life and excitement. There's often an implication of "insubstantial," or even "silly" when a person or idea is described as frothy.