angle attribute in android gradient

Gradient basically represents the variation in space(in a direction) of any quantity. With color it represents the variation of color intensity in a direction represented by angle. Here are some diagrams to represent this concept:

Here the figure shows the color variation in horizontal direction (angle is set 0).
XML code:

    <shape xmlns:android="http://schemas.android.com/apk/res/android">
    <gradient
        android:startColor="#000000"
        android:angle="0"/>
   </shape>

Here the figure shows the color variation in vertical direction (angle is set 90).
XML code:

<shape xmlns:android="http://schemas.android.com/apk/res/android">
<gradient
    android:startColor="#000000"
    android:angle="90"/>
 </shape>

You can also use different color as start, center and end colors. The code you attached contains all these elements.