This quote suggests that the person being referred to is a valuable and deserving individual in a romantic relationship. The phrase "she's a keeper" implies that she possesses qualities that make her worth holding onto. However, the speaker states that the recipient failed to keep her, implying a lost opportunity. The quote goes on to express a sense of regret, stating that she will now be with someone who truly understands how to treat her well. Overall, it highlights the significance of appreciating and valuing a partner, as failing to do so may result in losing them to someone who can provide the care they deserve.