It is especially important for operators to be able to gauge the health and accuracy of what is displayed at a glance. In a highly distributed system like Ignition, it is especially important, as the client may be located at quite a distance (maybe across the world) from the physical process it is monitoring and controlling.
For these reasons, components will get visual overlays for various reasons to indicate that the data they are displaying is not good. Each data binding that drives a component is evaluated for quality. If any of these qualities becomes poor, the component will show an overlay. The different overlays can mean different things, denoting their underlying cause. What they indicate is based on the Quality properties of Tags.