REDDIT EXODUS

Reddit announced in late May, 2023 that they would be making an initial public offering to investors and would begin charging third party apps to use the Reddit API. This API is what allows the third-party apps to talk to Reddit and actually function with the content of the website. Despite reassurances to developers, the resulting cost was prohibitively expensive and effectively forced these developers to shut down their apps.
Highly popular apps such as Apollo, Reddit is Fun, and others were considered the lifeblood of Reddit to many; most people who did anything beyond casual scrolling of r/all hated the official app and modern desktop interface and exclusively used these third-party apps. This change also killed off many useful, load-bearing moderation tools used by many subreddits.
Ultimately, this led many users to abandon Reddit in favor of a better website. Lemmy was one solution presented by those who had already found it, with the slightly younger kbin right behind. These platforms are part of the "fediverse", a collection of websites using a shared protcol called ActivityPub to communicate and share text and image content.
These apps diverge from traditional centralized websites in that they can intercommunicate via the shared ActivityPub protocol, but are also distributed among many servers or instances and federate together. Since much of the ire toward Reddit was focused on the singular cadre of ivory-tower-dwelling owners, this model was very appealing to many abandoning Reddit.