Mastodon is an open source, decentralised social media platform. Similar to other social media platforms, once you create an account you can create posts, communicate with other users, follow their feeds, and so on. You can read up on it further for more information.
Mastodon has a simple API that allows you to write code that accesses Mastodon actions in various ways. An API (application programming interface) is a way for two computer programs to "talk" to each other. Put simply, you can write a CircuitPython program that interacts with Mastodon to do things like send a post, or read all posts under a certain hashtag, and much more!
The first thing you need is your Mastodon API access token, which is necessary to communicate with the Mastodon API. The next page covers how to get your token. Then, there are two CircuitPython examples: posting to Mastodon, and tracking all messages using a particular hashtag on Mastodon. All of this using your Raspberry Pi Pico W!
This guide assumes that you have a Mastodon account. If you do not, you can visit the Mastodon site to sign up. If you would like more information before doing so, check out the Mastodon documentation.