I am new in Kafka and want to understand what is the difference between AsyncAPI and Kafka itself?
AsyncAPI is a specification, bringing async semantics to (synchronous) OpenAPI models.
Apache Kafka is a distributed event platform. It has a TCP protocol, but otherwise no external "API specification". All events sent to Kafka are binary, not types like cloudevents.
You can implement the AsyncAPI around Apache Kafka...
As per docs
The AsyncAPI Specification is a project used to describe and document message-driven APIs in a machine-readable format. It’s protocol-agnostic, so you can use it for APIs that work over any protocol (e.g., AMQP, MQTT, WebSockets, Kafka, STOMP, HTTP, Mercure, etc).