What is the meaning of xmlns:tools in Android XML layout?

It defines the XML namespace of the document. You should put it, otherwise tags like <RelativeLayout> could be not recognied by the parser.

Namespaces are a way for XML documents to include tags from various vendors. By using xmlns attribute you declare, that, by default, you're using XML elements defined here: http://schemas.android.com/apk/res/android (note that this link is broken - this discussion explains why).

You also declare additional namespace, tools, which is not your default namespace, thus when referencing elements or attributes defined there, you must add tools prefix, on example:

tools:context=".SomeActivity"

Following is a useful link from Android dev portal: https://developer.android.com/studio/write/tool-attributes.html

It says

Android Studio supports a variety of XML attributes in the tools namespace that enable design-time features (such as which layout to show in a fragment) or compile-time behaviors (such as which shrinking mode to apply to your XML resources). When you build your app, the build tools remove these attributes so there is no effect on your APK size or runtime behavior.

i.e. tools namespace helps designing UI and all attributes with prefix 'tools' will be removed at build time.

In fact, when you do :

<RelativeLayout android:id> </RelativeLayout>

Instead of calling android:id, the xml will call http://schemas.android.com/apk/res/android:id . It just the page that declare all the attribute and views that you can use in your xml.

Here is an explanation.http://www.w3schools.com/xml/xml_namespaces.asp