This is a Basic Procedure tutorial page. It explains how to perform a single procedure which is required in many different contexts.
A dynamic array property works like an array property, in that it displays a subtree of numbered items. But a dynamic array usually starts off empty. The Display → Skins property is a good example.
When the property is active, two widget buttons appear: Empty and Add. Clicking Add creates a new entry at the end of the dynamic array. Empty would remove all entries.
This entry works just like any other property. In this example, the property type is a texture, so it has buttons of its own to set the resource property. There are also the Delete and Insert buttons, which delete the selected array entry or insert a new entry above the selected one respectively.
Chip: Is "extend" the best word for the title, since it's similar to an Unreal Script keyword? Granted the context is very different, but it might be confusing for beginners. How about "populate"?
Wormbo: This page will only rarely be linked from coding pages and mappers usually don't care about UnrealScript-related things like words being similar some UScript keywords.