What does "composite panels on a frame" mean?
Traditionally, aircraft are built of frames (analogous to a person's skeleton), as shown below, and the smooth outer skin of the aircraft consists of panels fixed to these frames.  There are many frames and, for example, the small size of the windows is because they have to fit between the frames.


Image source: Wikipedia

The A350 uses this traditional construction, except the frames and panels are built of composite materials (carbon fibre reinforced plastic), rather than aluminium. However, the fuselage of the Boeing 787 seems to consist of a composite "tube", with reinforcing ribs running the length of the plane. There seem to be many fewer "ribs" running around the fuselage in this design than in traditional ones.


Image source: Wikipedia