what is xmlns tag for xacro?

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what is xmlns tag for xacro?

Here is the tutorial of xacro

It is confusing for me to see the below code

<robot xmlns:sensor="http://playerstage.sourceforge.net/gazebo/xmlschema/#sensor"
       xmlns:controller="http://playerstage.sourceforge.net/gazebo/xmlschema/#controller"
       xmlns:interface="http://playerstage.sourceforge.net/gazebo/xmlschema/#interface"
       xmlns:xacro="http://playerstage.sourceforge.net/gazebo/xmlschema/#interface"
       name="pr2" >

Why linking to those paths?

When should I use this?

How should I write this?

I need a detailed explanation to it. I do not know what xmlns value I should assign to.

Thanks