Images are way easier than walls of text to explain how uMatrix works. So let's visit wired.com.
When you first visit wired.com, open the matrix:
The top-most left-most cell is the currently selected scope. The currently selected scope is just a user interface thing. This allows you to look at, or edit rules in a specific scope. The matrix filtering engine always filter net requests by evaluating rules in the narrowest scope first.
All rules in uMatrix are created in a specific scope. Above in the picture, the * is selected. The * is the global scope. The global scope contains rules which applies everywhere, on every page you visit.
Since uMatrix works in block-all/allow-exceptionally out of the box, pretty much everything is blocked in the global scope, except for CSS-related and image resources.
Let's switch scope to wired.com: click the scope cell, and select wired.com. The matrix looks different now:
Here we see that in the wired.com scope, net requests which originate from within the wired.com domain are all allowed. (Except for cookie, that is my personal setting and I forgot to remove it before taking the screenshots.)
This is because of the 1st-party row: the cells in this row are used to tell what to do with net requests which are 1st-party to the URL address of the current page.
[TODO: complete this walkthrough]