Mastodon is a federated social media platform ( Actually a microblogging platform similar to Twitter ) based on a communication protocol called Activity Pub. It was released on 16th March 2016 and was created to be a safe place to share Ideas between people without having to trust a single organization for all your data. It is a free and open source software and can be used to self host social networking service so that you have the option to keep your data with you or with whom you trust.
The greatest feature of mastodon is it's federated nature. Mastodon doesn't have a single server that store and moderates the data coming in and going out from it unlike Facebook, Instagram, Twitter or any other centralized platforms have. Which gives you the option to choose who to trust with your data and why.
Mastodon has many servers with their own moderator(s) all around the globe hosted by different organizations with different mindset and different regulations. These servers are known as Instances which can be categorised by the topics,places, ethnicity, etc. So they don't have a single set of rules and regulations but have different rules on different instances making communication between known and unknown communities easier.
Different instances have different populations and are moderated by different moderators which makes it easier to reach the users and respond to their issue. If a instance has 1000 peoples and two moderators, a moderators should only have to moderate 500 peoples, since the moderators are also the active members of the community who constantly interact with what is happening in the community they can solve the issues of the users in a more systematic and timely manner. On the other hand Twitter like platforms mostly use AI to moderate and the issues might take days or even weeks to get responded. There are times when people are responded for their issues by their moderators within minutes in mastodon.
Since it is based on Activity Pub anyone can communicate with anyone on the other instance or even with other platforms like pleroma and peertube( based on Activity Pub ) from a single mastodon account without having to make different accounts for different platforms in order to communicate with the people in that platform ( Like you need a seperate accounts to follow users in different platforms like Facebook, Twitter, Instagram, YouTube ,etc. )
Mastodon includes a number of specific privacy features. Each message has a variety of privacy options available, and users can choose whether the message is public or private. Public messages display on a global feed, known as a timeline, and private messages are only shared on the timelines of the user's followers. Messages can also be marked as unlisted from timelines or direct between users. Users can also mark their accounts as completely private. In the timeline, polls can also be made and you can also create messages that can be displayed with an optional "content warning" feature, which requires readers to click on the content to reveal the rest of the message. Mastodon servers have used this feature to hide spoilers, trigger warnings, and not safe for work (NSFW) content, though some accounts use the feature to hide links and thoughts others might not want to read.