What is mock test class in Salesforce and is it important?

To deploy or package Apex, 75% of your code must have test coverage. By default, test methods don’t support HTTP callouts, so tests that perform callouts fail. Enable HTTP callout testing by instructing Apex to generate mock responses in tests, using Test.setMock.

example :-

public class CalloutClass {
public static HttpResponse getInfoFromExternalService() {
HttpRequest req = new HttpRequest();
req.setEndpoint('http://example.com/example/test');
req.setMethod('GET');
Http h = new Http();
HttpResponse res = h.send(req);
return res;
}
}