Transparency / Gradient / Opaque / Opacity
You can overlay one image onto another so that the foreground image does not completely obscure the background. Here are a couple of ways this can be done.
One way is to discard the background so an image like a logo or a person can be overlaid onto whatever background or colour is required. PowerPoint slides and web pages are typical examples of where you might want to do this. In this example the logo has no background and overlays the grey lines without obscuring them. This is not really transparency but amounts to a similar effect.