A 9-patch image, also known as a 9-slice image, is a stretchable image used for certain UI components like the dialogue box, item container windows, and item tooltips. The image is “sliced” into nine parts: four corners, four borders (top, bottom, left, right) and the center area. The four corners remain static, but the borders will be stretched and the center area repeated to dynamically accommodate any content that is being displayed within the box.

Default Dialoge Box Image:

In-Game Example: