The Apache module is broken up into several sections to address different aspects of an Apache configuration. On the main page, these sections are grouped into Global Configuration and Virtual Servers groups. Virtual servers is the method used to locate several web sites with unique domain names on a single IP address, in order to conserve the rapidly diminishing IPv4 address space. In the context of the Apache module, Global Configuration refers to configuration information that will apply to all virtual servers that are run from the same httpd daemon. It is usually unnecessary to run more than one httpd daemon on a single machine but it is possible. It is also possible to manage more than one such daemon with Webmin via the module cloning feature.
The official Apache documentation is a good source of additional information.
Global Configuration is found on the top half of the divided Apache module page. It provides access to the various options that will be shared across all virtual servers, such as which modules to load, network addresses, MIME types, etc. The options configured here will apply to every virtual server and the default server. To configure an option for one specific virtual server it must be configured in the virtual servers configuration section.