It's a 2016 game. It has cutesy pixel graphics. That's literally all I know, but it has been spamming my frontpage for two weeks now, despite me not being subscribed to this community.
If you're seeing it and you're not subscribed here there you must be browsing Local or All rather than your subscribed communities.
Subscribed - only stuff you're subscribed to (called Home in some apps).
Local - all posts from communities on your instance, and this Stardew Valley community is on your instance.
All - all posts your instance knows about, everything from your Subscribed and Local feeds plus every post from every community that someone on your instance has subscribed to, including communities on other instances
So the answer is that you're seeing a lot of posts because someone is making a lot of posts, people are upvoting them (the game is from 2016 but it's still very popular and still getting new content updates) and you're viewing feeds where they show.
It's a great game, I assume if this community is showing up it's just because there is active discussion going on? I can't tell if you're asking a question about Stardew Valley, or if you're really asking how Lemmy works?
I think the community is new? So lots of people saw it and started making their own posts too. When I started a community I spammed it with posts because I wanted people to find it