What are the notify-keyspace-events in Redis

I started to setup spring project by following this spring-boot article. But suddenly application could not start successfully, instead I am getting the following stack trace:

Exception in thread "main" org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'enableRedisKeyspaceNotificationsInitializer' defined in class org.springframework.session.data.redis.config.annotation.web.http.RedisHttpSessionConfiguration: Invocation of init method failed; nested exception is org.springframework.dao.InvalidDataAccessApiUsageException: ERR Unsupported CONFIG parameter: notify-keyspace-events; nested exception is redis.clients.jedis.exceptions.JedisDataException: ERR Unsupported CONFIG parameter: notify-keyspace-events    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1566)    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:539)    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:476)    at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:303)    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:299)    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:194)    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:755)    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:757)    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:480)    at org.springframework.boot.context.embedded.EmbeddedWebApplicationContext.refresh(EmbeddedWebApplicationContext.java:118)    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:686)    at org.springframework.boot.SpringApplication.run(SpringApplication.java:320)    at org.springframework.boot.SpringApplication.run(SpringApplication.java:957)    at org.springframework.boot.SpringApplication.run(SpringApplication.java:946)    at demo.UiApplication.main(UiApplication.java:29)Caused by: org.springframework.dao.InvalidDataAccessApiUsageException: ERR Unsupported CONFIG parameter: notify-keyspace-events; nested exception is redis.clients.jedis.exceptions.JedisDataException: ERR Unsupported CONFIG parameter: notify-keyspace-events    at org.springframework.data.redis.connection.jedis.JedisExceptionConverter.convert(JedisExceptionConverter.java:44)    at org.springframework.data.redis.connection.jedis.JedisExceptionConverter.convert(JedisExceptionConverter.java:36)    at org.springframework.data.redis.PassThroughExceptionTranslationStrategy.translate(PassThroughExceptionTranslationStrategy.java:37)    at org.springframework.data.redis.FallbackExceptionTranslationStrategy.translate(FallbackExceptionTranslationStrategy.java:37)    at org.springframework.data.redis.connection.jedis.JedisConnection.convertJedisAccessException(JedisConnection.java:195)    at org.springframework.data.redis.connection.jedis.JedisConnection.setConfig(JedisConnection.java:618)    at org.springframework.session.data.redis.config.annotation.web.http.RedisHttpSessionConfiguration$EnableRedisKeyspaceNotificationsInitializer.afterPropertiesSet(RedisHttpSessionConfiguration.java:170)    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1625)    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1562)    ... 15 moreCaused by: redis.clients.jedis.exceptions.JedisDataException: ERR Unsupported CONFIG parameter: notify-keyspace-events    at redis.clients.jedis.Protocol.processError(Protocol.java:113)    at redis.clients.jedis.Protocol.process(Protocol.java:131)    at redis.clients.jedis.Protocol.read(Protocol.java:200)    at redis.clients.jedis.Connection.readProtocolWithCheckingBroken(Connection.java:285)    at redis.clients.jedis.Connection.getStatusCodeReply(Connection.java:184)    at redis.clients.jedis.Jedis.configSet(Jedis.java:2797)    at org.springframework.data.redis.connection.jedis.JedisConnection.setConfig(JedisConnection.java:616)    ... 18 more

The main message is: ERR Unsupported CONFIG parameter: notify-keyspace-events.

What does it mean and how to fix it?