A web slice test is a type of test in Spring Boot that focuses on testing only the web layer of an application, without starting a full server or requiring a real database. To isolate the web layer from other dependencies, such as services or repositories, mocks can be used to simulate their behavior and return predefined responses. To create and inject a mock for an existing bean in the application context, the @MockBean annotation can be used on a field or a parameter in the test class. This annotation will replace any existing bean of the same type with a mock created by Mockito.