The HttpCalloutMock interface enables sending fake responses when testing HTTP callouts 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.

Now that you have specified the values of the fake response, instruct the Apex runtime to send this fake response by calling Test.setMock in your test method. For the first argument, pass HttpCalloutMock.class, and for the second argument, pass a new instance of your interface implementation of HttpCalloutMock, as follows:

Test.setMock(HttpCalloutMock.class, new YourHttpCalloutMockImpl());

After this point, if an HTTP callout is invoked in test context, the callout is not made and you receive the mock response you specified in the respond method implementation.

Useful Documentation