To give a general rundown of the "tiers" with further elaboration:
25 Subs: Does nothing. It's literally just there so that new creators don't despair looking at the progress bar that goes to a hundred.
100 Subs: You can unlock Ad revenue. The banners are pretty small and unintrusive, so I can't imagine a lot of people click them, which is likely why the views generate so little revenue. I mean, we've seen people on the forums who say they don't even see the ad banners... That's not exactly amazing ad real estate. (If you want to make money on ads, I'd recommend putting your comic on a website or similar and setting up with comicad or similar. Ads aren't the biggest earner on Tapas). Honestly though ad banners haven't been a big earner online in over a decade.
250 Subs: You can unlock Ink donations and revenue withdrawal. This is where most of the money you'll earn on Tapas, at least until you hit tens of thousands of subs, will come from. Engage your audience and take part in Inksgiving and this can pay off well (Lifetime income on Errant from Tapas so far has been over $700, which isn't bad for a comic of its size).
500 Subs: You can get a merch shop. When you make a piece of merch, it has a base cost. Then you set what your profit on top of that base cost is. When somebody buys a piece of merch, you get whatever amount you set the profit to. I've sold just a few shirts. I can't say they're a great way to make an income because the whole shop integration is a bit hidden away, and shirts haven't been how webcomics make money since the "t-shirtpocalypse" happened many years ago (I'm serious, this was a real thing that happened in webcomics. Merch and ad banner earnings suddenly imploded).
SECRET (not really) HIDDEN TIERS!
At 10,000 or 50,000 subs, you can apply for the Creator Bonus Program or the Early Access Program. On the Creator Bonus Program, creators get a lump sum sent over if they hit certain monthly view targets, and on the Early Access Program, readers can pay ink to unlock episodes a week early.
Now, it IS possible for Tapas to invite you to one of these early, like they did me. I was put on the Creator Bonus Program at 1600 or so subs. BUT, honestly it's really hard to hit the view targets at those kinds of numbers. There's a reason the required subs are set so high.
To genuinely make a full-time living on Tapas, you really need to be probably over 50,000 subs, and to also have a Patreon, and I'd also recommend periodic Kickstarter campaigns for print books and similar to supplement income. Conventions can be good too (again, if you have print books to sell and good merch, and if you can attend cons without spending too much money to eat into the profits) Most creators I know in the 10,000 sub region have a part time job to supplement what their comic brings in.