Tags enable dynamically storing information on an element that isn't read via screen readers. React native uses these to store the test identifiers. See this appium discuss thread

http://developer.android.com/reference/android/view/View.html#setTag(java.lang.Object)
http://developer.android.com/reference/android/view/View.html#getTag(int)
http://developer.android.com/reference/android/view/View.html#findViewWithTag(java.lang.Object)

It is generally preferable to use IDs (through the android:id attribute) instead of tags because they are faster and allow for compile-time type checking.