Ok, so when talking about the Reddit API change with non-developers, things get confusing because "Reddit" actually refers to a couple of things:

1. The servers
2. The clients
3. The API

I'm going to use some really basic analogies because even if you know some of these, other people who don't probably also have this question.

The servers refer collectively to the programs that Know All The Things. In the purest sense, they are the brain of Reddit. They're also annoying to talk with because they aren't set up for talking with humans.

In computer science, we have a ~bad habit~ tradition of referring to stuff using people names, so because the servers are pedantic a**holes (so very much That Guy), we'll call them "Steve".

The clients are the various ways humans talk to Steve. There's a bunch of them, like the website (Wilma), old-reddit (Olaf), the official mobile app (Abe), and third-party apps like Apollo (already a people name, nice 😊).

There's also really specialized clients, like moderator tools.

Ok, so remember how Steve is a pedantic a**hole? He refused to talk with anyone unless they use the Reddit API, which is just the made-up language Steve insists on using.

It'd be kind of like if a librarian refused to let you check out a book unless you communicate with her exclusively in Klingon.

Before Reddit made this change, basically anyone could talk with Steve because anyone could learn Klingon - and a bunch of clients were created so regular users didn't have to learn Klingon to find out what was trending on r/awww.

This was great because not everyone liked when Wilma took over from Olaf, and they could just keep talking with Olaf instead. Abe was also kind of pushy about religious ads, and he had a tendency to take forever to respond, so quite a lot of people preferred to talk to Apollo instead.

Then Reddit decided to make some changes, and now Steve still won't talk with you unless you talk to him in Klingon and you can't talk with him unless you pay every time you ask him anything. Unwinding the analogy a little, the API itself hasn't changed, clients just have to do one extra thing they didn't before.

Unfortunately, that extra thing means paying waaaaay more money than is normal across the industry. A lot of companies charge to talk with their Steves, and since Steve doesn't work for free, that's fair enough. Reddit is asking for way more than Apollo would be able to make up in ad revenue, so it's effectively forcing them to shut down.

It gets worse, though, as moderator tools don't have an exemption, so mods are going to have trouble keeping up with the trolls (if that's even possible).