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. :)

In addition to what others have said, PieFed is written in Python, rather than the highly difficult to learn Rust language, so new features are added roughly weekly rather than basically yearly.

Despite having been around for a LOT less time than Lemmy, it has already mostly eclipsed it in terms of features that people keep asking for. Examples include categories of communities, which are now user customizable and shareable, merging of all comments across all cross-posts (still distinguished which they came from though, and a link to their OP) to save those clicks and thus help defragment the Threadiverse, hashtags, etc. And those democratization of moderation features that I linked... even Reddit does not offer that!!!?!!! (it's goal being to make money, rather than offer an experience that people actually ask for and want to use)

Caveat: it does lack "polish" in many areas, as it is still catching up to Lemmy and Reddit (even as it surpasses them in some important ways). e.g. there is a Preview feature available when making posts but not for comments, and like Notifications will often fail to function properly e.g. point to deleted posts rather than be removed.

What will GREATLY help with the above is there are two apps adding support already. Interstellar is already available in the Play (+App I would presume?) Store, though still alpha level so also not polished yet entirely functional and the dev (as too with PieFed itself) very responsive. The other is a fork of the well-known and regarded (and FOSS) Thunder, but still only accessible via GitHub atm.

So TLDR of this TLDR: PieFed is a Lemmy/Reddit replacement that is in many ways better than those platforms, although in some ways not entirely perfected yet.

Personally, I use PieFed as my daily driver for 99% of Threadiverse tasks. I fall back to Lemmy rarely for things like its superior (atm) search functionality. Use whatever you enjoy, though I do recommend at least walking through the account creation process (even if you delete it later), bc it's really heartening to see that account creation wizard and how welcoming it makes coming to the Fediverse, whether from Reddit or Lemmy!:-)

It's another software compatible with Lemmy

https://piefed.social/ to see what it looks like