... for different reasons: when they're feeling vulnerable or flirty.
Not good: her hands winds round her throat or covers it (back of hand facing you) and stays in that position. You've offended her physically or emotionally and "gone for the jugular" - her hand flies into position literally to protect it by forming a barrier. The message to you is: stop whatever it is you're doing or talking about, backtrack and reassure her.
Good: stroking her throat, neck, or upper chest sends a definite erotic signal. Our throat and necks are "lovers-only" zones - no-one else gets to touch or kiss us there. If it's combined with a neck-arch, this signal is even more potent - by titling her head to further expose her throat to your view she's hinting she's starting to trust you.