Mindfulness in design is about a pleasing awareness. In relationships, it can mean infatuation. It's knowing that this is good, that this makes me happy. It's satisfying. You feel it in your body.
Flow is from work done by Mihaly Csikszentmihalyi. In his book, Flow, Csikszentmihalyi describes a state that people enter when they are fully focused in which they experience immersion in a task or activity to the point where they lost track of time. As the user, you sense no friction between you and the design on the way to reaching your goal. You may even spend more time than you planned because you're having fun, or being productive, or both.
Meaning comes from a feeling of fellowship, contributing, and making the world a better place. It's about harmony and, as Norman says, reflectiveness—because good things happen to you, you want to do good for others. As a user, you reach your goal quickly, easily, and happily, but you realize you're involved in something bigger than yourself, that your involvement is making a positive difference in others' lives.