Parts of a texture can be see-through, so what's behind them shows on the model. WebUVs shows it three ways.
An opacity map is a second image for a material, in shades of gray: white is solid, black is clear, and grays are partly clear, as glass is. An MTL file names one (map_d), as does a .3ds or .i3d material. Open the image and it goes to the material that names it, listed under its texture in the Textures section.
A texture with clear parts of its own, such as a PNG with an alpha channel, shows them clear on the model. Its wholly clear parts hide nothing behind them.
An .i3d material with a "!" anywhere in its name keys out one color of its texture, showing it clear: the first color of a paletted PNG, or pure black in any other. That's how such models cut out leaves, fences and windows.
See-through parts show on the model in the 3D View, with the File preview and Textures on. The UV Layout shows the textures as they are.