严格意义上来讲，这个不算是springboot的特有功能，仍然属于spring部分的功能。
先看下这个接口的定义：

/**
 * Extension to the standard {@link BeanFactoryPostProcessor} SPI, allowing for
 * the registration of further bean definitions <i>before</i> regular
 * BeanFactoryPostProcessor detection kicks in. In particular,
 * BeanDefinitionRegistryPostProcessor may register further bean definitions
 * which in turn define BeanFactoryPostProcessor instances.
 *
 * @author Juergen Hoeller
 * @since 3.0.1
 * @see org.springframework.context.annotation.ConfigurationClassPostProcessor
 */
public interface BeanDefinitionRegistryPostProcessor extends BeanFactoryPostProcessor {

    /**
     * Modify the application context's internal bean definition registry after its
     * standard initialization. All regular bean definitions will have been loaded,
     * but no beans will have been instantiated yet. This allows for adding further
     * bean definitions before the next post-processing phase kicks in.
     * @param registry the bean definition registry used by the application context
     * @throws org.springframework.beans.BeansException in case of errors
     */
    void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException;

}

即实现postProcessBeanDefinitionRegistry方法，可以修改增加BeanDefinition。
此特性可以用来动态生成bean，比如读取某个配置项，然后根据配置项动态生成bean。
举例：

public class Datasoure {
    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

@Configuration
public class DataConfig {

    @Bean
    public BeanDefinitionRegistryPostProcessor beanDefinitionRegistryPostProcessor(Environment env){
        return new BeanDefinitionRegistryPostProcessor() {
            @Override
            public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException {
                BeanDefinition beanDe = BeanDefinitionBuilder.rootBeanDefinition(Datasoure.class)
                        .addConstructorArgValue("datasoure1").getBeanDefinition();
                registry.registerBeanDefinition("soure1", beanDe);
            }

            @Override
            public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {

            }
        };
    }
}

以上postProcessBeanDefinitionRegistry方法中可以通过env来读取配置项，根据配置项来进行datasoure注册过程，此处代码未实现此种逻辑。