The best way to let someone know you understand and care about what they are saying is to

actively listen to them. This means giving them your full attention, maintaining eye contact, and nodding or engaging in non-verbal cues to show that you are actively processing and understanding their words. Additionally, you can provide verbal affirmations such as repeating key points they made or summarizing their thoughts to show that you are truly listening and comprehending what they are saying. Asking questions to seek further clarity or expressing empathy and support for their feelings can further demonstrate your understanding and care. Overall, the key is to be fully present and responsive to their words, emotions, and thoughts.