xmln:tools的作用

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 xmlnsattribute 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"