What does `collapseContentDescription` of toolbar do?

In Android Toolbar, I found app:collapseContentDescription but don't know what is it and how to use it?

<android.support.v7.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            app:layout_collapseMode="pin"
            app:collapseContentDescription="Collapsed" />

Any explanation? Thanks