Cats communicate with humans through purring by using it as a form of social bonding. When a cat purrs while being petted or held by a human, it can indicate that they are feeling content, relaxed, and comfortable in the person's presence. Purring can also be a way for cats to seek attention, express affection, or communicate a need for food or other forms of care. In general, purring is a multifaceted communication tool that cats use to interact with their human companions in various ways.