Mastodon is a federated social network, an alternative to Facebook and Twitter that gives users more control over their data. As with other social networks, you can send pictures, video, links or text. But unlike other services, you are in complete control of what you see; there’s no algorithm running in the background with control over your feed.
Rather than being a single service, there are many independent communities that users can belong to (or start their own); users can send messages within or between communities. The default community is at https://mstdn.social/, but you can choose which community to use for your primary identity. You can later change communities without losing your followers.
On top of that, Mastodon doesn’t serve any ads, is built on top of open-source software, and is funded through donations rather than venture capital or selling user data. The donation-funded model certainly has its risks, but hopefully it remains viable for a long time!