In "Watership Down," the quote you’re referring to touches on themes of trust and reassurance, particularly in relation to vulnerable beings like rabbits. When Fibber mentions that to make a rabbit feel safe, you say phrases like "It’s all right. I’m your friend," he is emphasizing the importance of creating an environment of safety and comfort.
Rabbits are prey animals, and they are naturally cautious and skittish. In threatening situations, they seek reassurance to alleviate their fear and anxiety. By using comforting language and expressing friendship, one can help a rabbit (or any fearful creature) feel safe and secure. This sentiment reflects larger themes in the book regarding community, friendship, and the need for supportive bonds, as the characters navigate their challenges in the wild.
The underlying message is about empathy and the necessity of creating a supportive atmosphere for those who are vulnerable.