@fembot --
First off, you need to understand what it means to be "Open Source."
This is a "Class" of software denoting software for which the original source code is made freely available and may be redistributed and modified.
That's it. After that point, most Open Source software lean on specific licensing. Open Source software utilize "Copyleft" and "Permissive" licensing, mostly.
Mastodon utilizes GNU Affero General Public License. https://en.wikipedia.org/wiki/GNU_Affero_General_Public_License
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@fembot --
I am not an expert on Open Source or Licensing associated with it, but my understanding is that the creators of Mastodon used the GNU Affero GPL because it specifies that it is intended for software designed to be run over a network, adding a provision requiring that the corresponding source code of modified versions of the software be prominently offered to all users who interact with the software over a network.
The network bit is important considering the purpose of the software
@fembot -- There is nothing to prevent someone from maliciously modifying the code, but it should be open and accessible for someone to review the code. The idea is that if there is nefarious or malicious intent, it could be found and reported.
There have already been instances of Mastodon servers losing licensing. (Trump's Truth Social in October 2021 for example.)