Blocks administration
From MoodleDocs
The blocks configuration page allows you to manage installed blocks and their settings.
Instances
This number tells you how many times a particular type of block is used in the courses on your Moodle site.
Multiple
Some blocks can be added multiple times to the same course. This of course is supported only by blocks for which it makes sense. For example, it makes sense to have two HTML blocks (and configure each one to show something different), but it doesn't make sense to have two Administration blocks. For those blocks which do support multiple instances, the administrator can choose to allow it or not.
