Here's how it's described on GitHub:

PieFed

A Lemmy/Mbin alternative written in Python with Flask.

• Clean, simple code that is easy to understand and contribute to. No fancy design patterns or algorithms.
• Easy setup, easy to manage - few dependencies and extra software required.
• AGPL.
• First class moderation tools.

Project goals

To build a federated discussion and link aggregation platform, similar to Reddit, Lemmy, Mbin interoperable with as
much of the fediverse as possible.

In short: You know how different Lemmy instances can communicate with each other? That's not limited to Lemmy - they can really communicate with any website running software willing to speak their language.

PieFed is another such software, along with Mbin. So it's basically a Lemmy alternative. It has different features: No mobile app support or alternative user interfaces yet (though it's coming), but in return other useful features such as keyword filters and feeds.

The fastest way to learn about it is probably by visiting piefed.social and checking it out yourself. :)