In an API, a payload refers to the data or information that is sent by the client in a request to the server or the data that is returned by the server in response to the request. It is the body of the HTTP request and response message. The payload can be sent or received in various formats such as JSON or XML. In a query string, the payload can usually be identified using curly braces "{}". The payload is used in the context of a message to distinguish it from the overhead data, which is the identifier used to show the source or destination of the payload. The overhead data is required only in the transmission process and is removed from the view once it reaches its destination. The payload is essential information in the data block that you send or receive from the server. The payload within an API can be used to update a resource, partially update a resource, or delete a resource.