A cliffhanger paywall is a paid break placed at the tensest moment of a story rather than at a natural chapter end. You read for free until the scene turns, and then the app asks for coins.
Why it works
Because a decision made while your pulse is up is not the decision you would make at the start of the chapter. Every element is tuned for that moment: the free run is long enough to make you care, the chapters shorten as the tension rises, and the price is shown as coins rather than money so the comparison with other things that cost two dollars never happens.
The economics are plain. One chapter is cheap, the book has three hundred of them, and nobody is shown the total.
How to read around it
Three habits help. Work out the per-chapter price before you buy a coin pack: pack price divided by coins, times the chapter cost. Check how many chapters the title has, because the number is usually on the book's page and almost never next to the payment screen. And check whether the story is finished, since an ongoing serial has no total to calculate.
Some platforms sell a subscription that unlocks everything instead. Where it exists it is nearly always cheaper for a heavy reader, and the app rarely suggests it at the moment you are most likely to pay.
Where you meet it
Mostly in serial-fiction apps, the ones that advertise thousands of romance titles. Subscription services and free archives do not need the technique, which is one practical reason to prefer them if the format annoys you.
See also serial fiction and the glossary.