Most software is limited by how much data it handles, how complicated that data is, and how fast it changes — not by how much arithmetic it does. That is what data-intensive means, as opposed to compute-intensive, and it describes almost everything you will be paid to build.
The vocabulary in this module — reliability, scalability, maintainability — is so worn from overuse that it has nearly stopped meaning anything. Half of this module is spent putting the meaning back, because you cannot design for a property you cannot state precisely, and you certainly cannot tell whether you achieved it.
It is a short module and there is no code in it. Do not skip it. Every argument in the three modules that follow is settled by an appeal to one of these three words, and the arguments go badly when people are using them differently.