Draw a horizontal line, then draw some short verticals that connect to the horizontal. The verticals are branches. Try to think that you put a short pulse into one end of the horizontal line (your "bus"). The signal will propagate through the line. Now you will also know that at every end of the line you will get some level of reflection of the input signal. This reflected signal will propagate back through the line. The shortest branch - with length zero - will not produce any reflection of importance, but as the length increases the effects may become noticeable.