My understanding is this:

Following a Lemmy community will add that community's posts and replies to your Mastodon feed.
Replying to a Lemmy community post or reply comment from Mastodon will generate a corresponding Lemmy reply comment.
(if I'm not mistaken) Making a non-reply Mastodon post that contains the handle of a Lemmy communtiy will make a Lemmy post within the associated community.

I'm open to being corrected about any of the above.