So you would have to sign up for a Mastodon instance to log on to the Mastodon app (I.e. Mastodon.social, mstdn.social, mstdn.es, etc). Similar to the fact that you're signed in to a Lemmy instance right now on lemmy.fmhy.com there are other Lemmy instances such as lemmy.ml, lemmy.world, Beehaw.org, etc.
Your Mastodon account would be completely separate from your Lemmy account.
Does that make sense? Not sure if I explained that we'll.
It IS a Twitter-like site. It displays posts in more of a Twitter style. That being said - it is also part of the Fediverse, so theoretically you should be able to see all federated communities on any fediverse instance of your choosing.
Personally, I have a kbin.social account and a mastodon.social account. I'm doing this because Mastodon doesn't really present threads from kbin or lemmy in an easily digestable way, so I will use kbin as my reddit replacement, and Mastodon to replace Twitter.
@CodingAndCoffee So you're saying I can reply to this from Mastodon also?
EDIT: please reply if you see this from kbin/lemmy 😅
Posting from mastodon.social, character limit here is 500. Definitely don't recommend this instance for a personal blog.