The Preview, at the top of the Textures section, picks what's laid over the UVs: the model's own textures, or a pattern, colors or numbers that show how the UVs lie. It shows in the UV Layout, under the islands, and on the model in the 3D View with Textures on in its toolbar. The UV Layout names the one showing over the square's top right corner.


Until a model has UVs, only Base can be picked: the others need UVs to lay out by.
How strongly the texture, pattern, arrows or numbers show in the UV Layout, over its background, and in an SVG saved with Include Texture Preview. It doesn't change the model in the 3D View.
What it does depends on the preview: