An extension is an extra feature that can be available from external sources. It can be a library, a theme, a plugin, or a tool. It can be anything that can be used to enhance the existing feature.
An extension is also can be created by the user for himself. By user-defined extension, the user can create his feature and use it in his project.
This user-defined or custom extension can be shared with other users. Other users can use this extension in their projects by importing or downloading it into their projects.
Likewise, blockysite also provides IDE for developing extensions. The IDE is a web-based IDE that can be used to develop extensions. For more information about the IDE, please visit the IDE page.