You won’t have all the features of mastadon with a lemmy account, but here are some things that can happen.
Mastadon users can post to lemmy and kbin communities. You can reply to these posts and both lemmy and mastadon users will be able to see it. For the mastadon users, the comments look like replies on a mastadon thread.
Mastadon users can also comment on lemmy/kbin posts. You can reply to those comments, upvote/downvote them etc.
Mastadon users can follow lemmy and kbin communities. But lemmy users can’t follow mastadon users yet. Kbin users can follow mastadon users. The reason kbin is less popular than lemmy is because kbin lacks mobile apps.
Side note: when linking to a community you have to do !community@instance. If you do !fediverse@kbin.social, it links to kbin.social’s fediverse community for everyone. If you just write !fediverse, that links to !fediverse@programming.dev for me, !fediverse@kbin.social for you and other kbin.social users, !fediverse@lemmy.world for lemmy.world users, etc.