Tube Frames are a part of the Redpower 2 mod and are a combination of a Pneumatic Tube and a Support Frame that lets you use the Tube Frame like a normal support frame and a normal Redpower Pneumatic tube at the same time. That is, the Tube Frame can be moved by Frame Motors and can also function as part of a Pneumatic Tube network at the same time.
There is also a redstone tube version of this called the Redstone Tube Frame.
One example of when this might be useful is if you want to move items from one end of a frame structure to the other end; i.e. a mobile mining platform.
Here is a tutorial video on Frames and Tube Frames and how to use them.