Retell AI is a powerful API that enables developers to build human-like voice agents swiftly and efficiently. By integrating custom language models, users can create voice experiences that can be deployed on phones or websites using WebSocket technology. Key features include ultra-realistic voices that convey emotions, low latency for prompt responses, and seamless interaction through interruption handling. The tool simplifies the creation of conversational interfaces, making it ideal for various applications like virtual assistants, customer support bots, and more.