"The audacity" is an exclamation expressing mock shock at someone's nerve. "She showed up two hours late, the audacity!" It is half-serious teasing, drawn from Black American slang and now used by everyone online. Not concerning, just dramatic reaction vocabulary. Often appears as the full phrase "the audacity of this man" or similar. If your kid uses it constantly, ask them where they first heard it, not to police their speech but to understand which online community they are in.