What is a reading list?
A reading list is a list of feeds you can subscribe to.
The author of the list can add or remove feeds. When they do, people who subscribe to the list are subscribed to the new feeds and unsubbed from the ones that leave.
I hadn't previously seen this described, but I've been looking for a way to share the blogs/feeds I follow on this site for some time (especially as I tried to figure out if a static site can operate as a microsub server — it can't really). So in this (slightly hidden, for now) section of my site you'll find a number of collections of feeds, all of which I follow and read when I have a quiet moment.
If you'd like to use them, you can find a page under this one for every feed I follow that includes the RSS feed and homepage of the site.
On the category pages you can also find links to 'OPML files', which contain links to all the feeds in that category — any good feed reader application (I use Reeder 5) should be able to let you import all of them at once.
Dave also mentioned an interesting quirk of 'reading lists' in his post:
Technically a reading list is exactly the same format as a subscription list. The only difference is you import a subscription list, and you subscribe to a reading list.
So if, like me, you use a tool like FreshRSS (which is superb, and easy to run with Yunohost and even a cheap Raspberry Pi) you can subscribe to a "Dynamic OPML" (the name that FreshRSS gives it), and your list of feeds will update as I add to them here (as well as the items in each of the feeds).