To all new folks comin' here:
You'll get more out of Mastodon by using unofficial mobile apps such as Tusky (Android) and Metatext (iOS) than through the official app.
You get a two more timelines to view: a Local timeline, and a Federated timeline.
The Local timeline consists of all public posts made by other people on the same instance/server you're on, while the Federated timeline consists of public posts including people outside of your instance.
You can the unofficial apps here: https://joinmastodon.org/apps
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Oops! Typo on the last line!
It should be "You can see a list of unofficial apps here".
I almost typo'd the above post too!