a Mastodon user joins a local pod much like diaspora, Friendica or Hubzilla. From this account one may connect to other users. To see those other users activity, one may view the stream from the "fediverse" of all Mastodon servers and some other pods - a stream that scrolls pretty fast, like a stock ticker; or one may view the local stream from one's own pod. Alongside these, one sees "notifications" from anyone who sends a direct message, and a "home" stream of content by one's contacts to all their followers.
On Mastodon, posts and comments are treated as the same thing -- comments are not inherently threaded under posts. So, even when you comment on someone else's post, it is treated like just another post on your timeline. you need to "tag" the original poster in your comment in order for them to get notified. If you comment on a Mastodon post from a Mastodon account, it automatically adds the @user tag when you hit the reply button. Users of Hubzilla and Friendica, which both communicate with Mastodon; become accustomed to tagging .
The local timeline (the feed of all users on the instance) features heavily in the Mastodon UI/UX (User Interface/eXperience. So, people tend to set up instances around common interests and then interact heavily with people on that instance. It saves the trouble of "following" and saves the wider audience of those users the constant barrage of posts about the focus of the locals. (thanks to email@example.com for this explanation)