When you @ mention a community from Lemmy as a user on Mastodon you can post to that community from Mastodon. The first sentence of your Mastodon post will be used as the title, which is why they often look so strange on the Lemmy side.
You can also follow a Lemmy community from Mastodon, but it gets a bit messy as every comment will be shown as a boost Mastodon side.
I hope the groups addition that Mastodon is working on will fix that mess.