When you say a computer platform has great scalability, what you are saying is that as you add more processors, nodes, memory, or some other piece of hardware (like disk), performance increases nearly linearly. For example, a system with great scalability might give you something like:
Processors 1 2 3 4 5 ... n
Performance 1 2 2.9 3.9 5.9 ... a litlle less than n
Poor scalability looks something like:
Processors 1 2 3 4 5 ... n
Performance 1 1.6 2.0 2.4 2.6 ... a bunch less than n