Pleroma is a social network microblogging server built on the open OStatus and ActivityPub standards. Servers running the Pleroma server software can exchange messages and user posts with other servers following those standards. Pleroma differs from other ActivityPub-compatible software in it's efficiency, lower system resource-use and it has a lot fewer dependencies.
The network of servers using the standard ActivityPub protocol are as a whole falled the Fediverse. Most of the Fediverse servers who take a pro free speech stance appear to be running Pleroma while those ran by very censorship-happy authoritarian administrations tend to use Mastodon. See the article Fediverse for a comparison of servers and their features.
A server needs to have Elixir and PostgreSQL in order to run Pleroma. That's it. It is therefore fairly easy to install and setup compared to alternatives like Mastodon - which require NodeJS, Rails, Redis, Sidekiq and a range of other software which makes setup and maintenance a nightmare.
You can find some general information about Pleroma at https://blog.soykaf.com/post/what-is-pleroma/