@SpringBootApplication - Spring Boot

@SpringBootApplication annotation indicates a configuration class that declares one or more @Bean methods and also triggers auto-configuration and component scanning.

The @SpringBootApplication annotation is equivalent to using @Configuration, @EnableAutoConfiguration, and @ComponentScan with their default attributes.

@SpringBootApplication Example

@SpringBootApplication annotation example: We use this annotation to mark the main class of a Spring Boot application:

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication // same as @Configuration @EnableAutoConfiguration @ComponentScan
public class Application {
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}

Read more about @SpringBootApplication annotation on Spring Boot @SpringBootApplication Annotation with Example.

@Configuration

Spring @Configuration annotation is part of the spring core framework. Spring Configuration annotation indicates that the class has @Bean definition methods. So Spring containers can process the class and generate Spring Beans to be used in the application.
Read more at https://www.javaguides.net/2018/09/spring-configuration-annotation-with-example.html

@EnableAutoConfiguration

@EnableAutoConfiguration annotation tells Spring Boot to “guess” how you want to configure Spring, based on the jar dependencies that you have added. 
Read more at https://www.javaguides.net/2018/09/spring-boot-enableautoconfiguration-annotation-with-example.html

@ComponentScan

This annotation enables component-scanning so that the web controller classes and other components you create will be automatically discovered and registered as beans in Spring's Application Context

References

https://www.javaguides.net/p/spring-boot-tutorial.html