What does @dimen/activity_vertical_margin do?
@dimen refers to dimension and it's a file where you define dimensions to use them later from in any layout file.
It's located in res/values/dimens. Here's what a sample of the file look like:
<resources> <!-- Default screen margins, per the Android Design guidelines. --> <dimen name="activity_horizontal_margin">16dp</dimen> <dimen name="activity_vertical_margin">16dp</dimen> </resources>Here activity_veritcal_margin = 16 dp.
and to use it like this:
<LinearLayout android:id="@+id/activity_main" android:layout_width="match_parent" android:layout_height="match_parent" android:paddingBottom="@dimen/activity_vertical_margin">Here we give this linear layout a bottom padding with 16dp.
@dimen/activity_vertical_margin or whatever @dimen/whatever_key_name is a reference to a dimension that probably is saved in your projectname/src/main/res/value/dimen.xml file
In android you can save several values for example dimensions, strings, integers, drawables...
Here you can find more information about it