“Open source” would be good. That phrase is now widely understood by the layman. From my point of view, however, people are unable to keep a thought. The shorter, the better in the marketing world, I would say.

I very much like the simplicity of “A discussion platform for communities”. If one wants to highlight the one feature that sets Lemmy apart from others, then adding “decentralised” would do the job for me. As others have already said, I am not using “federated” because that can only be used when “federation” is a known word, which it absolutely is not. If you understand “federation”, then you probably already know about Lemmy. So,

“Lemmy, a decentralised discussion platform for communities”.