As a tool to read the output:
Not entirely necessary when you're more familiar with it, but still useful. And when starting it's much easier to read.
https://www.depesz.com/tag/unexplainable/
This series of blog posts is one option. But my impression is that most of this comes simply with experience. At first the main thing you'd look for is simply whether it uses the indexes you want it to use, or if it does full table scans.
I'd also recommend to simply read the Postgres manual, at least the parts that are not entirely about the syntax of various commands. There is a lot of information in there that will help to understand how Postgres works.
I personally prefer https://explain.dalibo.com over depesz's version, both from a visualization standpoint (I can more clearly see the hierarchy of operations) but also because I've had some not great experiences with depesz the individual. Great blog, but a really soured opinion of him.
You may personally find the output awkward, but typographers will disagree. They didn't always have high density displays. They did always have superior type rendering, with output more closely matching the underlying design. Hinting was used, but they didn't clobber the shapes to fit the pixel grid like Microsoft did.