Translations:F.A.Q. - Frequently Asked Questions/65/en
Lemmy is the "reddit" of the Fediverse together with Kbin (discontinued). So it's a content aggregator service that can be used like a forum.
You can find a list of instances here: https://join-lemmy.org/instances
You can follow and take part in Lemmy discussions from other parts of the Fediverse.