Reddit used to have a free and open API. This allowed 3rd parties to develop apps / interfaces for the site. These apps helped everything from making the site usable for some with accessibility issues to blocking ads to providing a customized interface to tons of other things.
Generally this was done by taking the API feed and re-engineering it to allow the desired presentation.
In a move to make the company more attractive to investors before going public, Reddit changed that API to a paid model. This meant any developer of those 3rd party apps would now have to pay a not insignificant in most cases fee to continue their access to Reddit. As such, most apps closed down and a very small portion of us long time Redditors migrated to Lemmy/ the fediverse.