What is mock test class in Salesforce?

Provide an implementation for the HttpCalloutMock interface to specify the response sent in the respond method, which the Apex runtime calls to send a response for a callout.
global class YourHttpCalloutMockImpl implements HttpCalloutMock {
global HTTPResponse respond(HTTPRequest req) {
// Create a fake response.
// Set response values, and
// return response.
}
}