Let me provide two actual common examples, graphically.
A. Scaling of CPU frequency
Graph 1 shows the framerate (of a certain game at a certain resolution) as a function of the frequency of a CPU.
The red graph (ie GTX570 3-way SLI) shows that an increase of the CPU frequency brings an increase in framerate, meaning the CPU is limiting/bottlenecking the graphic subsystem.
The green and blue graphs has a zero slope, meaning the CPU is handling the graphic subsystem very well.
B. Scaling of multi-GPU
Graph 2 shows ''single GTX570 vs 2-way vs 3-way SLI''.
Theoretically, going from 1 card to 2 cards would bring 100% improvement in framerate (ie a factor of 2.0). In this actual example, it is 1.82.
Going from 1 card to 3 cards would bring 200% improvement (ie a factor 3.0) and here the actual number is 2.36.
:Graph 2 is basically reading Graph 1 along the vertical Y-axis at a certain value of x