<beans> to <beans:beans> - element beans must be declared - spring

Question

In the code below, why do I get "element must be declared" (From IntelliJ) if I change the start / end tags from "beans" to "beans:beans"?
What's the significance of the ":beans"?
<?xml version="1.0" encoding="UTF-8"?>
<beans:beans xmlns="http://www.springframework.org/schema/security"
xmlns:beans="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.1.xsd
http://www.springframework.org/schema/security http://www.springframework.org/schema/security/spring-security-3.1.xsd">
<!-- HTTP security configurations -->
<http auto-config="true" use-expressions="true">
<form-login login-processing-url="/j_spring_security_check" login-page="/login" authentication-failure-url="/login?login_error=t" />
<logout logout-url="/j_spring_security_logout" />
<intercept-url pattern="/login" requires-channel="https"/>
<intercept-url pattern="/backend/**" access="isAuthenticated()" />
<intercept-url pattern="/todoes/**" access="isAuthenticated()" />
<intercept-url pattern="/resources/**" access="permitAll" />
<intercept-url pattern="/**" access="permitAll" />
<remember-me key="mySecondSecretWordThatShouldBeHidden" user-service-ref="userAccountDetailsService" />
</http>
<global-method-security secured-annotations="enabled" pre-post-annotations="enabled" />
<!-- Configure Authentication mechanism -->
<beans:bean name="passwordEncoder" class="org.springframework.security.crypto.password.StandardPasswordEncoder">
<beans:constructor-arg name="secret" value="myVerySecretWordThatShouldBeSomewhereHidden"/>
</beans:bean>
<authentication-manager alias="authenticationManager">
<authentication-provider user-service-ref="userAccountDetailsService">
<password-encoder ref="passwordEncoder"/>
</authentication-provider>
</authentication-manager>

Answer 1

You have bound the security schema to the default namespace using xmlns=.... This means you can use the elements in that namespace directly without qualification, e.g. <authentication-manager>.
To use an element defined in another schema you need to bind that schema to another namespace and use that as a prefix. Declaring xmlns:beans="http://www.springframework.org/schema/beans" binds the schema identified by the URL to the namespace beans. The location of the schema is in the xsi:schemaLocation. Example usage <beans:bean>. Had you declared the namespace as xmlns:wibble="http://www.springframework.org/schema/beans", then this would change to <wibble:bean>.
You could have used any of them as your default namespace, which one makes sense depends on your config file and the types of bean it will have.