They are just a rendered image, usually a single item such as a figure, plant, table etc, that is saved as a .png file which has no background. Thus it can be copy/pasted to any background.

The term "Tube" actually comes from Paintshop Pro where a "tube" is an actual file format known as .tub. Images are imported to the Tube Tool and are then applied using that tool.  I only use the tube tool in PSP for things like creating strings of pearls or when I want to make strings of images.

The term also applies to any isolated image that has no background, such as an image where a portion of it was cropped out and saved on it's own without a background.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi