A file with .addin extension is an Add-in definition file created by Microsoft Visual Studio for add-in projects. It is used for registering new add-ins in the Add-In manager of Visual Studio IDE so that these can be used in other projects without re-creation. Add-ins itself are small software applications that extend the functionality of bigger programs by getting to the primary programs. Addin files are stored in XML file format at the same location where the add-in project is created.
ADDIN File Format - More Information
ADDIN files are saved to disc in XML file format that is human readable. It can be opened in popular text editors including Notepad, Notepad++, Microsoft Visual Studio IDE, and many others. Microsoft has defined the XML manifest file of an Office Add-in that describes how an add-in should be activated after it is been installed and used with Office documents and applications.