Start with Entry files and modules: it sets the rule every other page in this unit assumes, that a module does nothing until an entry starts it, and shows what that rule buys you when one feature fails. The next two pages apply it to a shared library and a typed one: keeping jQuery to one copy when a theme still loads it, and typing the data a template hands to a script, the one boundary where adding types pays off most.
The last three are about what the build ships and when: splitting a heavy library into a chunk that loads only on demand, setting a size budget the build can enforce, and bringing icons and fonts to the page without a blank screen or a layout jump.